If it is a compression problem, the only way to truely fix it is to increase the size of the compression volume (volume above the piston at TDC).
I had a 318 with a compression problem way back in the day. I had 10.5:1 TRW forged pistons and stock 318 heads. The engine would overheat after 20 minutes when the temperature was above 78° F. My Internal Combustion Engine class teacher told me that it was due to my compression being too high. I swapped the heads with 360 heads that had 6 cc more volume and never had a problem again - and was even able to run cheap pump gas!
If it is truely a compression issue. It is easier to swap heads to ones with larger combustion chambers, than to change the pistons. You can also consider using a thicker head gasket, but that may not solve the problem alone as well as a set of heads with larger combustion chambers.