Shouldn't be an issue finding air shocks or cross referencing them. All they need to do is get the full compression and full extension lengths of the stock shock (listed in any shock catalog), then find the air shock that has the same style ends and compressed and extended lengths. Even then, the lengths do nat have to be exact, as long as they're within about 1/2", they will work fine. I don't have the current catalog here, but I'll check the one at the shop to see if I can find some part numbers for you, as I'm certain that Monroe does make air shocks for the '55-'59 Chevy/GMC.
I do find it odd that you're compressing the springs enough to make the tires rub, though. I'm running a fairly large tire (30x9.50) on my 3200, with the stock 8 leaf springs, and have never had a rubbing problem, even with a somewhat heavy load. I've even bottomed out the rear suspension, and never got a rub. It sounds like what Bill (red58) stated, the springs are weak.
