I'd guess about 56 or so for a few years after that.
although I'm pretty sure tubeless wheels on a 1 ton were usually 19.5" early on.
I've seen 17.5" Chevy wheels with fewer lugs as often used on NAPCO 1/2 tons.
Can't say that I've seen 17.5" on a 1 ton Chevy, but would not surprise me in the least.
Grigg