The "rule of thumb" is 9.5 cr max with iron heads. With a mild cam and no quench, a true measured 9.0 would be safer. I agree with your engine builder, compression was high for pump gas. In some ways, a street driven car has higher detonation risk than a track only car. The engine is drawing engine compartment air which may be 150-160F on a hot day and it can be run for extended periods of time which heat soaks everything.

The only mystery is why no apparant detonation. The only thing I can think of is that it happened at higher RPM and was masked by the noise and power of the engine.

To fix it you have lots of choices. The ideal fix if you keep iron heads and the same cam would be quench pad pistons, matched to the chambers to give .040" quench with the setup dialed in for 9.5 or just a bit higher CR. This likley means new pistons and labor to dial in the quench.

The gasket deal may drop compression at lowest cost but make sure its enough to get you under 9.5 since there would be no quench.

Depending on the pistons you are starting with It may also be possible to mill the tops to lower CR. You would need to consult the piston manufacturer to determine how much if any can be removed without hurting piston structure.