Are you trying to run this thing on pump gas? Even so, I wouldn't be afraid to run 8~10psi with a 6-71 at 8.5:1. You could get there with the Probe's that I mentioned above. The top ring is .320" down from the crown as I remember. So not a bad choice for a blower piston...

With a splash of race gas, you could run even more if you wanted to "turn it up".