That one rotted away for a reason, chances are the rest of the them aren't in great shape either. If you can pull the engine out to do them all, then great but that one looks accessable to replace without pulling the engine out though. In the mean time you can put a rubber expansion in it to get you by for a while, these are available at any good parts store.
While that plug is out you'll want to flush all the rust and gunk out of the block.
A rubber plug wouldn't really be considered a permanent fix but would probably last the life of the car. You can use either steel or brass at this point because chances are the coolant wont be left to rot in that engine for another 40 years. These cars get pampered now,they don't get treated like disposable daily drivers like they once were.
