Yet another idea...

I know that it wasn't possible to weld cast iron years ago, but they're doing it now in cylinder and engine block rebuilding shops. Maybe you could take that manifold to a cylinder head rebuilding shop and have them weld a 1/4 inch steel nipple into the threaded opening you're having difficulty with. The wall thickness of the nipple might be too thin for welding, but I expect you can get 1/4 inch nipples in Schedule 80 and 120 for greater wall thicknesses if you special order them.