If you're doing serious towing or getting near 500 hp you'll want 4-bolt mains. There's no way to tell without pulling the pan and looking at the main caps because 2 and 4 bolts are both machined from the same block casting. If you're going for all-out power, it's better to start with a 2 bolt main block and have it machined for splayed main caps, but I don't know how much a machine shop charges for that.My block was from a '91 Chevy K2500, 2 bolt main but I know a lot of people suggest that a 4 bolt main engine is a better foundation to build on. I am not sure how to identify the 4 bolt main engines.