Can i safely run "6"-heat range spark plugs with my setup?

[dwadia]

I have a redtop in my 1995 240 which has several mods. It has a gt3071r, jwt s4 cams, 740 injectors, etc etc. I bought it with most of the stuff already done. The previous owner dyno'd it at 430+ hp at 26 psi. However, i generally run 12 or 15 psi and the MAX i would run is ~18 (very rarely). It's running very rich and the plugs are currently quite black. They are bkr7e's. I know the 7 heat range is necessary for 20+ psi, but at the levels i'm running can i try a bkr6e and see if it runs better? Would there be any risk with that?

Also, what should i gap them to? And is there any relationship between heat range and what the gap should be? If so, please tell me what the gap should be for both a 6 and a 7. Thanks.