Bazza's 318 is running (!!!), but....

Yeah, that's the intake manifold exhaust heat crossover, sorry for the confusion. I suppose it could technically be an exhaust leak too but not in the traditional sense like from a manifold or pipe. I'm not sure if the crossover is considered to be part the exhaust system? Maybe it is somewhere, I don't know.

It's obviously related though in that it employs exhaust heat to warm the intake manifold faster on cold starts. It is supposed to work in conjunction with the heat riser mechanisms on the exhaust manifolds that always get rusted stuck.

It makes sense that the choke well rusted out because it is exposed directly to exhaust heat on cold starts which would create condensation. Over time the moisture would eat through the bottom of the well like it did on yours.

I'll shut my trap now, seems like you have it figured out.