If you DO decide to retain the stock heads and want to do a little port work, then the exhaust ports of a stock GM head is what you should go after first because they suffer the most in terms of power. Do a little clean-up short side work on the intake side and you should be good to go.

And if you're looking for someone who is good at porting heads it would be "JimV" (616) 889-0928. He is known for getting the most out of a set of heads for hi-perf marine applications and put Bob Madara on the cam.