SBF's have a peculiar little situation. If you install the head gasket in the wrong position it blocks some of the cooling holes. If one of your head gaskets got put on backwards that could be your culprit.

There is an "F" on the gaskets, make sure that goes to the front. I don't know if that is what you've got, but this screw up has happened to all of us in the rush to put an engine together, and the problem is, the gasket fits either way.

Don