Hp limit?

My wallet is small. Just starting to gather parts for my 68 Dart. Doing a b motor 4 speed. Would like 450hp for a street/strip car. I have orig. manifolds but I know I'm giving up hp with these. Headers and the 4 speed is a tight fit. So is there a combo that will get me to the 450 range with the manifolds and iron heads or should I bite the bullet and go alum. heads with headers? I'm trying to keep cost down. Have the rest of the car to build. ( body,interior and so on). I have a 69 383hp but it has a 400 cast crank that hasn't been cut yet. Any help on this would be great. Thanks!