Thanks for the post. I was just kind of confirming my suspicions, but as the old saying goes if you hear it twice it might be true and if you hear it three times it must be true.
Anyway, as for the fibreglass tank, the trick is to mount it differently than how Kenny mounted it and to vent it, at least that is my hope at this point. The 'All Alloy' is really rarer of the two, as there were only 6 made. It's 'All Alloy' VR is my favorite, although the two T160s are pretty close. It's actually an addiction, bikes, cars, you can never have too many.
It must not make much difference if you use a 52% or a 63% balance factor with a dry crank. After all, L.A.B. has a 1973 manual that shows 52% dry and I have a 1975 manual that shows 63% dry. (I assume the ‘73 manual refers to the same 850 engine as shown in the ’75 manual.) What’s more, as MichaelB pointed out, component matching is actually more important than the balance factor in reducing engine vibration.
That was the 1973 06 5146 manual I got the 52% dry info from, dealing with all models from 1970 to 850 Mk 1 and it doesn't make any distinction between 750 and 850 as far as the balance factors are concerned, so whether the 52% figure applies to both models I don't know or whether it was something that was overlooked when the manual was printed?
My 1975 manual certainly gives the 63% dry figure.
Personally, I would concentrate on matching the components, especially the rods. If you purchased quality forged pistons, their weights will not vary much. A triple beam balance scale works well for weighing components. And a dermal tool can be used to remove material.