Best way to reduce compression ratio?

I apologize because I know I've told this story a million times. When I built my 408, 93 octane gas was available and I naively thought it would remain available, so I built the engine with 10:5.1 compression. I used forged pistons and had the reciprocating assembly balanced. The pistons are either at zero deck at TDC or maybe a little above, I can't remember. I'm running some ported "J" heads on the engine.

It ran pretty well on the 93 octane, but it detonates like crazy now that I can only get 91 octane, and I am afraid I am going to damage the pistons, so I would like to reduce the compression to 9.5:1 somehow without replacing the pistons.

I'm using a stock Fel-Pro head gasket which is already pretty thick, I believe, so I doubt I'd get much of a C.R. reduction from a thicker gasket, assuming one existed.

Any suggestions for me, or am I "SOL"? Thanks, FABO buddies.