it depends on what your going to be doing with the motor, if its just going to be a drag motor, and only go a quarter mile at a time, then the polishing will help out. but really, if its a street motor, polishing can actually hurt your performance. in your combustion chamber and in your ports, there is small pours in the metal, all of those little raises and indents will actually help absorb some of the heat from the combustion, and cool the air going into the chamber. when you polish it, all that heat is just reflected back so you might have problems with hotspots, or dentonation. if you do a good port job, match your intake and and header to the head, and smooth out and open up alot of the intake and exaust runner, you should get a pretty substancial gain. despite what you have heard, you can actually make more power with a castiron head then with a aluminum head. the castiron takes longer to heat up, and disapates the heat more efficiently then aluminum which spreads the heat throughout the entire head. the major reason why alot of racers use them is wieght. the 5 extra hp that you could have with the cast iron, is about equal to the small amount of wieght that you trim off.

if your bored 60 over, and your only looking at a little over 350, you must be running a low compression ratio. im looking at that number with a stock compression stock pistion 350 with a big cam intake header and carb. with a good vlave job, and some porting, nothing major, its not going to help that much unless if you got high compression pistions, a huge cam, and a big carb. but if you have all that, then i wouldnt even mess with stock heads in the first place. i would buy a nice set of dart or edelbrock heads.