Actually, I want to keep the FB as stock as possible. The link I posted regarding the ramairrestoration headers would require additional changes to the exhaust system as I’m aware of now. So I prefer to find a LH exhaust manifold to add to my replacement RH side one, which has been lying dormant for 6 years.
I meant the drain plug thread and there are time sert kits available. However, depending on previous work on the thread you may need different kits - this can’t 100% be decided before removing the pan. I know there was a flange available for C2 Corvette oil pans facing the same problem: GM #3723008 or you can make it by yourself: 1/4”- 3/8” flat steel about 2 x 2” and a thread 1/2-20 in the center, which is then welded in the pan. This is something I can prepare in advance.
Since at least one stud of the LH manifold is broken and it needs to be drilled out I was told the engine has to be removed. Most probably more will shear off while removing the manifolds.

Oliver