Original GM wheels are the same bolt-pattern as FORD and DODGE = 8-on-6-1/2".
GM center-holes have always been about 1/4" too small to clear the FORD or DODGE hub, as GM wheels have always been hub-centric, whereas FORD and DODGE wheels were lug-centric.
Most aftermarket wheels dis-regard GMs hub-centricity and simply make one wheel with a center-hole that fits over all, which is why many GM owners are never really quite satisfied with the ride quality of aftermarket wheels.
So far as the 16.5s go, I would not have them as a free gift; they are more trouble than they are worth, even if they were plentiful.