I had a tunnel ram that was rectangle port, and oval port heads on my 396. I was told to use rectangle port gaskets and it'd be fine. The coolant passages are in the same spots, so that isn't an issue. The engine builder I was using at the time told me the same thing you typed. He said it wasn't a surprise to see rectangular port manifolds on oval port heads straight from GM on different BBC's. I can't vouge for that as I never tore any apart in the late 60's early 70's. He sure had. I don't know if anyone has ever heard of him. Maybe you have Jerry? Larry Laveretty? He used to have several different race cars and raced at Cordova a lot.