The problem I ran into when I was using stock pistons and also the l266f's with a decent size cam, is the lack of valve reliefs. When I was working to get the compression up with milled down the heads and used a steel shim gasket, I had no piston to valve clearance with a moderate sized cam. Its been years since I built those motors but as I remember I had to walk back the cam and use a felpro gasket to get suggested PTVC, which lowered my compression again.