Question: what is the difference between a 2 bolt and a 4 bolt 350?
Answer: Two bolt means each main cap uses two bolts to hold it down. Four bolt means there are four, which means more clamping force, and less likelyhood that the crank is going to push itself out of the bottom of the engine at high RPM / power. Here is the "splayed" 4-bolt mains. These are always aftermarket.