Go with the 140!
I have an '86 GSXR750 Ltd., and I'm running Dunlop 591's. Those are still available in a 140/70-VB18, and that's plenty of tire for that bike!
As the others stated, your choice of tires will be limited, but I raced in the AMA National Supersport series in '88-'90, using those same 591's. While obviously not up to today's tire standards, they are still a pretty decent tire.
The hot set up for the older GSXR's is to use the rims off the '88 GSXR1100. That bike was the original first-generation body and frame style, but for that one year, they went to a 3-spoke wheel, with a 4.5" x 18 rear rim. And best of all, those rims bolted right on the other GSXR's with no modifications.
Unfortunately, those particular rims are very hard to come by, as ever since, Suzuki has used only 17" rims, and most of those 18's have been grabbed up by now.
Of course, you can always go with aftermarket rims, up to 5.5" x 17.
But for the stock 3.5-18, use the 140 tire...