If that was the case couldn't you just buy a higher pressure rad cap?
Im having same issue did head gasket solve the problemUPDATE: I went ahead and tore the engine apart last night. I can't really tell for sure but I'm assuming there was one water port that was getting pressurized from the compression. I was worried my steel sleeve slid further down into the cylinder but to my surprise it never budged. I'm still waiting for the new gaskets and rings to get here. This time I'm going to coppercoat the head gasket and torque the head bolts to 42 foot pounds as per other members suggestions. Hopefully this should solve it.
My one observation was for how short of a time this engine has been ran before being torn down again, there was a lot of carbon build up on the piston crown. I'm not sure if I can attribute that to possibly running rich or the leaded aviation fuel I am running OR maybe that's normal?. Back when I was running pump gas and nitropropane, the piston crown had very little carbon on it along with a nice brown surface. I'm debating whether or not to drop the compression back to 12.6:1 by using the 3 layered gasket or whether I should stick with the single layer gasket. I'm not sure whether or not I will be able to tell the difference power wise between the 2 compression ratios.