[Rachelsdad]

Quick follow-up to all of this:

As it turns out, the leak was coming from the mating surface of the exhaust manifold. A couple passes on the surface grinder and voilà! No more exhaust leak.

The '63 really wasn't meant to use flange gaskets, apparently. The output port is rounded, and the pipe should just be drawn tight to it (assuming it is smooth enough to make a proper seal, and of course, a stainless exhaust pipe is going to be tough to get "form fitted" to the manifold).

Anyway, all's well that ends well. No substitute for truly flat mating surfaces.

Thanks for the Remflex pointer, though, guys. I've got their site bookmarked for future reference.