340 with Magnum heads

Shouldn't be any problem at all. Many guys do this conversion all the time. Depends on what piston you use as to what compression you will end up with. If you choose your pistons right you won't have to worry about running race gas. Like Bob said his cc'd at 67. Most I've seen go from 62-67 cc. I've heard some guys talk about having problem finding good ones but I've had 2 sets and none of them were cracked.