Source for 273 Oversized Pistons?

Something to keep in mind. The factory advertised stock compression ration on a 273 2bl was 8.8 to 1. Given your specs on the motor ie deck height, chamber column, etc. you could get a CR around 9.0 to 9.3 or 4 by going with the flat top and a thin gasket of around .028. Perfect CR for todays fuel. I’ve done that several times with 273s.

Also, try and find some 920 casting heads. Best flowing head for the 273. They came on late 65 through 67 273. Next option would be the 302 casting swirlport head.