I'd think you could be close to 400 w/o the heads. Headers, intake good cam and a tune and I'd bet you'd be really close to that. My old 67 coronet (3850lbs) went 12.90 @ 104mph with a stock 70 440 rebuild (9:1 at best), Summit 488 cam, RPM intake 750HP carb, 1 3/4 headers, stock 906 heads with a fresh valve job. That's about 338hp at the rear wheels pretty close to 400 if not a shade more. If you plan on more mods I'd go w/ the eddy's if not just use some of that money to clean you haeds up and roll. And why buy expensive hi-flowing heads and choke them up with Manifolds?