Swapping a roller cam into an old block costs a lot. Machining a 2-bolt block for 4-bolt caps is cheap. Not to mention a 2-bolt block will take a good bit of abuse. I'm running a 2-bolt right around 430 hp and I don't worry a bit about it. 116k miles with no issues.
Roller cams are super nice, but retrofitting one is expensive and not worth it. Go with the roller block. If you need 4-bolt mains, have it machined.