Slant 6 exhaust leak.

After much thrashing I finally got my 73 Dart engine running decent. I have very loud exhaust leaks at cylinders 1 and 6. Quiets down after everything gets up to operating temp. Besides replacing the gasket do I get the manifolds decked at a machine shop? Could it be the head is not flat? I suppose once I have it apart I might find another reason for the leak. Any advice? I'm a slant 6 neophyte. Thanks.