Henry the American Indians say we don't fully mature until age 56 and I say once you're over the Hill You Pick Up Speed! I've recently learned an ole Irish saying when I was frustrated on recent Commando recovery, "We are not here for a long time just a Good Time. I merely add that I want to take as many with me as I can.
The Commando is the ideal bike for your end of the World ride quest. Regardless of what the naysayers say, there is no other bike that can be so comfortable to live on. Its vibes are so dulled down its more therapeutic than traumatizing fatiguing but for sure not if configured along the lines the vast majority here do low bars and hard sleek solo seat and tiny tank you can't rest on or grip with knees, pashaw. I've done just enough long distance to find my body and bike would completely disappear for effortless cruise in a trance state fully alert just a grin and eye balls projecting into the wind and future.
I'd get a '71 or '72 crank cases and a small port standard head with standard cam set up for mid 8ish compression ratio and a single 30 or 32 mm carb. Mikuni should last longer than an Amal.
The frame is fully up to as much off road rough stuff as you can take, no one breaks frames except by removing the wonderful isolastic mounts. Alls you should do is just weld a 1/2" dia tube brace about mid way down front frame and another across the middle of rear frame up-bends. Add the simple 3 rod links
and you will not believe how competent the handling and isolated from nuance road and wind jiggles wiggles and upsets so even if feeling low and ill its refreshing to be on the Commando. No fork damper as not needed and takes away from the light steering, like English racer bicycle. All other modern bikes of significant size and power are sluggish dull brutes compared.
I have solved the Roadholder fork issues and inexpensive kit is now available for better fork action than I've experienced in sport bikes, even modified-upgraded or endruo, trials and MX bike, poor dangerous corner cripples on or off pavement, pashaw.
The front frame tubes are too close to stretch a leg out on to rest, but a bar across the fork brace [you should add one] allows foot forward ease and also is a protector of bike when dropping all loaded down. Saved me more than once away from home as bars and controls and tank and mirror and signals were fine.
After a few miles of feeling what the forks are doing you will be way more expert on what is really happening and matters most over the most elite cycle engineers-designers for poor dangerous corner crippling ignorance, pashaw! I"m spending way too much on Ms Peel to have power and lighteness to out sprint the moderns as already know they are dangerous corner cripples at rates and loads Ms Peel is a breeze to take, plus 3 more faster way funner ways to change direction they don't even know a cycle can do w/o crashing, heheheehehehehhehhe
I'd stick with steel rims WM2 on both ends to fit same size tire but would go with 18" as way more common than 19". I'd get a dual tread tire that's more on road oriented as most traction is by tire compound and inflation pressure, not tractor cleats which roughen ride and deform on pavement crisis dodges.
I'd put on 2" taller rear shocks to match the 2" taller stance my fork mod can provide for more road shock and hi centering clearance, like going over edge of a wash out or peaks in the path and more cargo capacity w/o bottoming. My fork mods can not be bottomed or topped out to even feel them but for soft sudden change of direction w/o any upset to chassis or pilot teeth.
I'd put front crash bars, attached up near the stem and down by the engine mounts and factory cross brace and triangulated so fork foot bar collides with them just as reaching the stem stop to both back up the fork twisting off and bending bars into tank plus keeping some the full load off frame tubes holding the main crash bars. So top of main bars would angle steeply down to clear legs out on fork tube and lower bar less up angled to meet at apex the fork bar contacts on crashing. You will go down by your own fault or others or who knows what.
Rear crash bars would be a squarish loop I'd configure to help attach big hard bags. Later can discuss how to do the hard attach points and some optional triangulation that should not hinder pilot getting out the way but could just as well hang on to bike tightly and not be hurt much either.
With my above mods there is A LOT to be said and confidence to stay on bike and fight out a horrific upset and save the day, but of course can also be worse as may be flung off in hi side bike can land on you, ugh. But can also stay in bedroom or office desk and not live much at all. I left the big city and big clinic practice for more time than money but an real cool unpredictable life style in rural Ozarks Dixieland. I'm impressed no end by your insight to use that endearing term for south side of Mason Dixon line.
I'd stick with 530 chain and start out with the X-ring sealed kind but 530 plain chain is very common around the world, just carry extra master links. Test wear by how much you can lift chain are back of sprocket out of the valleys as that is how far up the chain links ride on teeth under way. About a 1/3 up valleys and I'm pensive to replace it as the teeth will soon be thinned the fanned over and breaking off. duh don't ask, I didn't come to my blunt opinions the easy way.
Bob Patten has been a mentor to me on the rod links and what happens to teeth in Gravel conditions which is the most common pathways world wide. He had to make a sprocket teeth to get back home. What I've found is dry chain best in the grit or makes a messy grinding paste. Expect about 10,000 miles before its a loosing state of more wear than economy not to switch chain.
I'd put on LED running and signal lights and a upgraded halogen or even HID headlight. Then can go slower and still charge to run. Biggest battery you can stuff in is wise as can ride longer if a wire breaks before finding fixing.
There is Sparx 3 phase alternator that increases wattage at lower rpm.
A LED color change or other volt meter is way worthile to keep tract of charge vs discharge time and alerts to charging and battery state.
I'd put in the lowest ratio 1st gear you can find and the taller 2nd gear ratio 850's use and gear box sprocket 21 T for long legged easy mileage loafing cruise. Most you fun accelerating and dicing the fun twisties under 80 mph will be the wonderful 2nd gear, but don't want to stay in low gears long as oil can't get into the sleeve bushes so eats them up which wobbles clucth basket and then miss shifts. 4th gear lock these part together so no oil no problemo and is good mid 30 mph to over the ton. I love the torque so I don't have to peddle shitf my Cdo around like I do my corner crippled, sluggish effort steering fat tired tingling vibration road racer modified '00 SV650, pashaw...
Interstate tank with a big tank bag on it you can rest your trunk/head on while still seeing ahead well is wonderful on the long boring endless opens to get somewhere worth being.... Any octane you get should work but may have to tweak timing a bit now and then for elevations and fuel available.
You will need aftermarket air filter as factory air box is trauma to deal with. I'd get the one piece kind and then over wrap it with filter foam to be easy to knock off most the layer daily and not have to deal with the main filter as much.
To carry heavy loads the rear loop needs struts from back of Z-plates up to luggage rack itself or just the rear of frame loop. Also double as handles to lift and drag up from spills. I'd somehow tie these to the rear crash bars and storage bag supports.
I'd go with a 2 into 1 exhaust as lighter and more low down spunk but also think about rigging a basket-holder on muffler for on the road cooking oven.
I'd not put on a full crusier fairing as heavy and wind buffet handling uspsetter but would a tall fork mounted screen you can look over w/o wind blast and maybe with lowers to keep blast off the legs some. The one in my photo's dampened semi truck blasts and stablized to self centering dampened forks at speed.
Pazion SureFire ignition seems most robust for the buxs, points may work but will be a constant attention getter on every slight let down on firing. Extra spark plugs of course.
As much as ya can mount fenders and brackets with thin rubber under which does dampen a good bit of the metal ringing most don't even notice they are noticing until done like that I found.
Tools and spares are a whole 'nother post while ya chew on the above and others pick apart my recommendations on what I'm doing on my mostly still a Commando where it matters most - Ms Peel. I plan my asses to be stored in her tank hung up in some bar or bike shop. How about you?
Looking like my headlight here, white patch is gel coat knocked off but still serviceable until a deer knocked it so hard the hinge broke so got another, my 3rd in a decade now, ugh...