Cranking pressure should not exceed 165 psi for operation on pump gas according to Crane Cams.

You may have shot yourself in the foot with your piston choice. If there is no large flat area on the crown to mate up with the underside of the head, then proper squishing of air/fuel across the chamber at TDC could be a problem. Here's a 12cc D-Cup from Keith Black that has the proper configuration. See the large, flat area on the right side of the photo. If your pistons have only a thin ring of crown material around the perimeter of the piston, there may not be enough area to form proper squish.....
http://kb-silvolite.com/performance....tails&P_id=155

A tight 0.035" to 0.045" squish is one of the best ways to prevent detonation, but again, there must be enough material to form a good squish.

At zero deck with a 0.040" gasket (8.65cc's with 4.100" bore), 12cc pistons and 64cc heads, the SCR will be 9.59:1

With 5.7 rods, 3.48" stroke, 9.59:1 SCR and the intake valve closing at 42 degrees ABDC @0.050" tappet lift, the DCR will be 8.103:1 and should support the use of cheapo pump gas, assuming the squish is right and the ignition system is optimized for the combination.