Does your 1975 Chevy truck have a 4 bolt or 2 main?

A 1975 Chevy truck could have several different engines, and the number of main bearings depends on the specific engine. There's no single answer. A small-block V8 would have a 4-bolt main, while a smaller inline-6 would likely have more than 2. To know for sure, I'd need to know the engine displacement (e.g., 350, 250, 292, etc.).