Nice Job and welcome.
My 72 combat is a work in progress much help found right here.
That is going to be a sweet Norton and you'll love the ride.
I shelved my orig. yellow fiberglass tank and side covers.
I saw white milky stuff in the fuel line and knew that would not be good for the pilot circuit.
I took it to a radiator shop and he washed out the inside of my tank to a brand new look.
He said he didn't like the fact that fiber strands in black were coming out still after many rinses.
I found an orig. metal tank and side cover set off ebay from a 72-73 era Norton. ($300 for a very nice tank stripped,$75.00 for side cover set)
I painted it yellow to honor the orig. color and have had zero problems.
You are doing a tremendous amount of work; to include engine work to have to worry about junk in the carbs issues.
Get a metal tank take the orig. to a great painter and let him find the right color match that's his job and they do it very well if they are good.
Keep it red as it's only fitting and enjoy many years trouble free riding.
Trying to play w/ the fiberglass tank is futile. It will last a couple years maybe more and then give you trouble.
A job done right once is a job only done once.
Again, stellar job on the old girl. I like the road tax badge still affixed on the seat rail. Classic!!!
Hope to save you a headache down the road my friend.
If ever in Penna area lets take a ride together
Marshal