Help with pre ignition ping

So a little update on my buddys issue. He talked with the engine builder and he told him that the pistons are custom made wiesco High compression pistons with valve reliefs. to get the compression "down" they used a .090 gasket and the X heads. to get it close to 10.5 :1 The motor ran better with less ping with 5 gal of 93 mixed with 5 gals of 100 octane av gas, however it still had some ping in it which makes me wonder if it needs more of a higher octane race gas to completely eliminate the issue, or maybe less 93 and more straight 100 octane. We know the ping isnt exceptable over a long period however what we're hoping to do is try to eliminate the issue with out a major tear down for the summer and then pull the motor in the fall and change to a actual 10:1 piston. thanks guys