1973 Dart 318 core plug replacement questions

Unless it's a special block it should just take the standard old core plug for a 318. Go to Napa or AutoZone or O'Reilly's and tell him you need to get a freeze plug for the side of your 318 block. Pop the old son of a gun out and drive the new one in. I like to paint mine with a couple coats of engine paint before I drive them in and let the paint act as a sealer.
You might notice a bunch of rust and crud behind the freeze plug once you pop it out. You need to flush that out that's one of the reasons your plug failed on you.