Leaks Because You Didn't Dress Mating Surfaces
Few things are as discouraging as leaks in a fresh restoration. Your Mustang's exhaust system has its own little demons. Jeff Lilly suggests dressing exhaust manifold mating surfaces as well as their cylinder head counterparts. Exhaust manifold gaskets do a nice job of sealing, but they can't do it all alone. When your Mustang's engine is built, have the machine shop shave all exhaust mating surfaces for excellent sealing. Make sure donut contact surfaces are clean too.