What kind of real world gains would I get by putting in a edelbrock rpm manifold on a 402 big block with a xe268 comp cam on a 750 q-jet. Hp gains and et. Also I have a 3.31 rear open end, would like to go with a 3.73 posi, You think there is a big difference in et?

I also saw a professional products manifold, are they a good company?

Thanks

That depens largely on what kind of manifold you currently have on the car right now.

it is stock, it is flat with no "rise" to it, looks like it would be perfect for a corvette.