Porting BB heads

Yeah, there's a ton of difference in those two castins. The 346 are open chamber and the 915 are closed. The ports are different too. Also, it can be said that the 1967 915 heads are actually the only real HP heads offered for the big block.....sans the Max Wedge stuff. I'm not countin the MP stuff.....just factory offerins. Have you decided on your piston yet? That should dictate which head you run. With the KB280 you could run either head, but you'll need to mill off the step head if you go with the 915 because you get the quench from the head there and not the piston.