Period aspect ratio should be 90% F+R (I had a Suzuki GS850GN back then), so your 80 front choice concerns me. It'll sit lower, ground more easily and the steering will be quicker !
Think OEM was 3.5" so closer to 90 but 100 would be fine. Guessing 110 would be too big for your newly-chromed guard.
If I recall, period rubber was Metzler, Pirelli and Avon .. why not ship from outside the USA (if you can't source) and tolerate the extra for the sake of the build?
90/90 x 18 is readily available- and since it has a 3.50 now, that seems the better choice I guess.
Cost of shipping from the UK is almost as much as the tyres cost!
No huge hurry, but I am just starting to look now in case I needed to order from somplace.