I heard once a long time ago that all of the GMC's came with 4-bolt mains, 'cause GMC only sells trucks, they would go a little out of their way to make sure they could justify someone buying a GMC over a chevy. FWIW.
In my blazer, with a 2-barrel original 350, it was a 2-bolt main.
Personally, if I were going to build a small block from stratch I'd prefer the 2-bolt because of
this. When you are doing a full rebuild on an engine, it is totally worth it in my mind for the added assurance that this little mod would provide. Granted, it's a 3-400 dollar upgrade, and maybe it's not really needed for a truck motor, but when you decide to go all out with the supercharger or nitrous, you sure wouldn't have to worry about the bottom end here.
Later,
Buddy