Oh boy, I hate to say it but youre going to have to change those heads if you want any power. You can probably get someone with a set of usable 516 closed chambers to trade you or sell yours to buy a pair. You will loose some flow but youre really going to need that compression, its going to be terrible.
Either that or have yours milled to about 80 CC, which is doable, youre just going to have to have the intake side machined as well.

I also like the 272 cam mentioned before, when I had my GTX it had a purpleshaft in it and Im not 100% sure which one but I beleive it was the 272 because it idled pretty tame. TONS of low end torque, half throttle and the tires were TOAST.

Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street