I have a 1998 Ford 460 engine with 48,000 miles. I have attempted 3 times to stop leaks in the exhuast mainfold (both sides leak). The first time the gaskets were replaced with original metal gaskets. The second time the manifolds were machined then double metal gaskets were installed. The third time a garlock gasket was used (this lasted the longest). I have been told that the manifold by Ford is a poor design. What can I do to make a permanent repair?