Would modifying the intake manifold to flow more produce more horsepower on stock heads? I've seen a few ported intakes and they claim to produce up to 25 more horses (probably BS). It makes no point of saying if different heads are needed, so I assume they are (if I wanted to see gains).

 

i would imagine the heads are better than the inatke. If you gasket matched the intake it would be more efficient. But i highly doubt you would see 25hp on a stockish setup.