Many thanks for the replies.

The complexity of the casting itself is not lost on me.

Financially speaking, I see this as a high-risk, zero-reward proposition. My eyes are wide open on that one.

But simply, as far as the cars are concerned, I think this is something that needs to be done. Were I the lucky owner of a brand-new '69 M-code dart, I'd have thrown those manifolds in the dumpster immediately, like most everyone else did.