Just fired my 400 engine after swapping on a set of 96 heads. The IM has a 1972 date code. There is a small gap at the exhaust crossover between the head and the top of the manifold flange. It's small enough I didn't even notice it when assembling, but there's an exhaust leak for sure. Is there a different IM gasket I can use, or some other remedy that someone has employed to fix this problem? I'm sure I've heard this discussed before. I just can't rememember what the fix is. Thanks