Adding to spark plug talk, maybe go to a standard tip plug. The BKR7E is listed as a projected tip. For starters and unless proven otherwise, I would start with a -8 heat range and a non projected plug.

if the piston top came off, ring butting is #1 thing that causes it, and #1 for that is too much heat for the gap set,or said other wise ring gap not set wide enough. Lean mixture, excessive spark advance, high compression, inadequately working cooling system, detonation and high HP per cubic inch all combine to increase an engine's heat load

Can you suggest a non projected tip 8 heat range plug? I had been looking in the NGK catalog, but don't think they make one. Maybe in another manufacturer?

414ci 623hp corr on dyno, figure 580ish corrected through the manifolds.

Also, I know I can go R5671A-8 (assuming that would be 1 colder than what I currently have) , but I was hoping for a resistor plug. I did run FR5 in a prior application so it should be fine. I wasn't efi then though, just concerned about the interference, but I have good wires.