Exhaust manifold

[hemi43]

Originally Posted by Mike C

I don’t think that’s an air pump hole… There would be one in each set of cylinders. Looks more like a manifold that has had the generator mount cut off. Does it leak exhaust out of the hole? I’d try a 3/8-16 plug and see if that would work.

It's a port for the EGR or air pump. No such thing as a 3/8-16 plug, and that hole looks much larger than 3/8" diameter.
It will be an oddball thread so IMO the best fix it to run a pipe tap in it. I would start with a 3/8" NPT or maybe even a 1/2" NPT.