freeze plug / coolant leak question 72 dart 318

I'll guess that the plug is rusted through after so many years so stop leak may not help. And besides, if the rad has any near-clgos in the tubes, they can clog closed with stop-leak. It won't be too easy; you have to get some working room. I would be pulling the trans tube and fitler and trying, but I'd be ready to detach the exhaust and remove the exhaust manifold. You can drill through them and then thread a lag bolt in for a puller. Or you can tap on one edge into the block and flip it sideways and them pull it out with pliers. But if you do that, be careful not to tap it into the water jacket and lose it inside. There are probably some other tricks that others can mention.

Reinstalling takes some room to be able to start it in straight and then tap it down. Dorman PilotSeal types are easier to start in straight. One alternative is a copper type expansion plug; Dorman used to make them in a few sizes.

See here on page 3: https://www.dormanproducts.com/catalog/hardware2006/127-130_Sec8_Part1.pdf

The rubber expansion types get used but I am personally leery of them.