You didn't say what manifold, i don't think the heads are too big, if you have a 454R intake and 2 1/8"/2 1/4" headers, we're pushing close to 750Hp with a pump gas 468" with Brodix 2 extra heads.
I'm guessing real close to 800 HP. with right tuning, maybe a little more. I'm buliding a 496" for a customer with Dart 308's steel heads and stacks injection, i'm looking to go more than 800 HP with it.
JMO
Zip.