general engine question

I get the feeling that my 318 isn't quite running how it should. It's a bit leaky and I really want to rebuild it over the summer, but that's not why I posted this... It seems like my carb is not quite jetted properly of the timing is a little off. If I get on it pretty hard and open up the secondaries, it dips the nose a little bit and gets real rumbly until it hits somewhere in the region of 4k RPMs (but that's an untrained ear guess). It'll pickup after that and pull pretty hard until I need to shift and the process repeats. it got way better when I switched from manifolds to headers, which seems to indicate that it's a carb deal. I have a 600 CFM Eddy carb on top of an RPM air-gap, the only other mod is headers and dual 2.5 exhaust.

I know this is a really vague question, not to mention one that is hard to diagnose without being able to get behind the wheel, but I don't know any car guys that could tell me what's going on. Thanks for any input you may have.