I wouldn't go rebuilding the engine if it doesn't need it. There's no reason a properly maintained engine from the 60's wouldn't go 100K plus without a rebuild. How's the oil pressure? Compression? How's it run? Smoke at all? If it has good oil pressure, doesn't smoke, and pulls strong, I'd leave it alone. It will end up being easier to pull the engine to deal with the freeze plug. You might as well just change them all. I'd install the brass ones, you'll never have to do it again.
