RPM vs. stock Manifold

RPM - no question.
As for cam, with stock stamped rockers you will likely run into interferences with lift over .480" even though the duration is in the range you want.
Maybe consider the CC 20-309-4 which is 222/226 and .464" vl. 3.55 gear if you are running a 26-27"dia tire.
If you go to the TTi headers you will probably add another 25hp to your combo - food for thought.
Get a bowl hog cut to get rid of all the material that is left after the 3 angle cuts and then blend into the port. Gind a bit of the material away from the push rod pinch point to improve air flow and try and make sure the head intake port entrance is a tad larger than the manifold outlet port to prevent any port alignment obstruction.

Any pics?