I could use a 1974 slant six manifold heat control valve expert!

Casting numbers (found on parts) are not necessarily the same as part numbers (found in cattledogs). Yikes, I do not think I would have put the money and effort into ceramic-coating an old manifold; if I were bound and determined to have ceramic coating for whatever reason (…?), I would have started with a new manifold. Too many failure modes for old manifolds—warp, crack, heat valve failure.

Fel-Pro (or other parts store) Slant-6 manifold gaskets aren't quite a guaranteed recipe for exhaust leaks, but almost—especially at the intake-to-exhaust "hotbox" junction. It is always very much worth your while to get the good gaskets for the manifolds-to-head and intake-to-exhaust junctions; R&Ring the manifold stack is a bіtch of a job, especially if your spine isn't 18 years old any more, and do-overs are even less fun. You are facing a do-over; there is nothing other than an exhaust leak that will cause a new "boomy" noise under the hood after manifold replacement.

To figure out that spring-unwinding thing you're seeing, check if the heat valve can be rotated against spring tension as previously described.