I'm probably gonna piss Richard off by disagreeing with him on this one. He could be right on the reduced piston dome and head gasket size, but I won't speculate because you never gave the specs on the kit.

Here is where I disagree. If you reduce the dome of the piston and thicken the head gasket (assuming the heads themselves were not milled or changed) you are effectively reducing compression. An increase in compression, an increase in timing or a decrease in the octane level of the fuel can all effectively contribute to detonation. So a decrease (reduced piston dome and thicker head gasket) in compression will not by itself contribute to detonation or pinging; quite the opposite will occur.

I think that the total combined advance of 42* that you are running with today's octane rated pump fuel is the problem. You can get your distributor recurved and get your vacuum advance adjusted and you will probably be just fine.