Suprised you had trouble with pistons. Arias and Ross for sure have the molds for the big chambers. For the latest 270 the chamber was the smallest available especially if you milled the head .075+". A flat top would be about right compression.

I bought some JE flat tops for a 270 with it bored for an available ring package since the 270 is pretty oddball on the bore many years ago an never used it. Stuck under my bench needing the small port head done. My plan was for a 50 Chev coupe that's now long gone, oh well. Good luck with your project........

216.158 MPH 12-Port 302 GMC on 70% 171.0 MPH 302 stock head on gasoline 7 years later