One other thought. Evan (Coilover) posted this some months ago. The discussion was about Remflex gaskets (which I will not endorse for a couple of reasons). Evan mentioned he used high temp rtv sealer...you can read/see here: https://tinyurl.com/yxnsycdb

I've been meaning to try Evan's method, but I don't think either of my manifolds is leaking.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end