Editing Chevy LS6 closed chamber heads

Found a LS6 (454) and put it in a 67 camaro. Its got the original closed chamber heads, 30 over, 11 1/4:1, Isky Z89 Cam shaft 630 lift - 304 dur, 108 seperation, victor jr, 900 carb, 2 1/2in exhaust, Hooker headers. This is the question. I had it dyno'd and it has 395 hp at the rear wheels. I was expecting a little more. Should I dump the closed chamber heads for different heads (I hear the open chambers flow better) or is there a way to improve the flow. I understand the intake ports are to big to flow well and these heads are keeping the hp down (and the 2 1/2" exhaust). AFR heads?

Hmmm, I thought all LS6 engines, which were produced first in 1970, were, in fact, open-chambered heads. So when you say 'original', it puzzles me.