Two suggestions:

1) Unshroud the intake valves some by enlarging the combustion chamber around them. Also take the sides of the comustion chamber out to match the cylinder bore - they ar generally a little smaller. This will give you a few cc's more chamber volume.

2) Carefully measure the piston's installed height vs the block deck at TDC, then use a HG to get a nice tight squish of somewhere betwwn .032 and .036 inches. This give a lot of beneficial turbulence in the chamber as the mixture burns (ignited some number of degrees prior to TDC, as it hits TDC the burning mess is snapped into the chamber from the edges) and do a lot to prevent detonation.

My PV's motor runs around 11;1 CR, and it's quite ping resistant, as long as I run premium (punmp 93 octane) in it. I can advance the timing until the power drops off without pinging, which is more ping resistance than a stock moor has at far lower compression ratio.
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'63 PV544 rat rod, '93 Classic #1141 245 +t