Cam lift is the issue for rockers. Most of the stock style stamped rockers don't like lifts in excess of about .510-.520. Spring rates are greater and you end up with pushrods punching through the rockers.

I'd home port the iron heads, lap the valve on the cheap or do a vavle job, mill and run them. Stock iron will get you easily to 400 with the right other items. Step up later to an aluminum head if you want with another engine. For what you drop on a set of aluminums, you can get the iron done and a juice unit!