Once you get the manifold off you will likely find that it's also corroded around the four water jackets where it meets the heads too, very common problem. Sometimes they get so bad as to leak coolant into the valley (which obviously ends up in the sump).
I've repaired a few of them myself over the years, just clean it up with a die grinder, fill it (fillet?) with a TIG up proud of the surface and then just carefully file it back flush.
Might be a bit harder around the thermostat opening, you'd probably require a mill to restore the lip that the thermostat sits in, but I guess it could be done by hand as well if careful?
These days it's probably only worth doing if you want to retain a "numbers matching" engine assembly, otherwise as Rexy says, just replace it?