blown 2 head gaskets

So far this season I blew two head gaskets in the same spot.The top of #2 cylinder, it blows into the motor and blows oil out every seal and hole in the motor. I have a 340 with the high comp. pistons, .030 over bore, Standard stroke, 360 j heads, six pack carbs and manifold, shooting 150 h.p. NOS, MSD set at 6500 max and removes 4 degrees during NOS spray. I have an rpm switch to turn off NOS at 6300 to keep from spraying on the limiter. I also have a WOT and fuel pressure switch to stop spray if not correct. Running about 6 psi fuel and 900 psi NOS. I use VP fuels 105 octane. The block is o-ringed to keep from blowing the gaskets??? I have ARP head studs not bolts.I looked into copper gaskets but not sure if that would help or cause a leak issue. I had the head shaved and the block checked, no problems found. The o-rings stand up .010 above the block. I am using FELPRO 8553 gaskets. No overheating, it happened on the first run of the day the last time.I am getting real good at changing head gaskets and would like to move on from this problem, any ideas would help.
Thanks Frank