In general, all GM non-metric wheels from 72 to present go something like this:
All full size (B and D body) cars, 5-lug full size pickups, astros and the like have the 5x5" pattern. All intermediate RWD cars (A, G, X, bodies), S10s, and AWD astros have the 5x4.75" pattern.
The problem is that Chevy rallies came in both patterns. Do you know which vehicle they came from?