225,slant six radiator

Try a good flushing first. Drain all the coolant, including the block plug mentioned. You may have to push a wire in the plug hole to break up rust get it to flow. Fill with tap water and add Prestone Super Flush. Follow the directions of driving ___ minutes and drain. That may clean the block well. Before that, I remove the top hose and put a cone of metal screen inside the hose to catch crud that would plug the radiator. Then take the radiator out and back-flush with a garden hose in the bottom, with it upside down. On a clean driveway, you can see the rust turds come out that were clogging the tube inlets at the top. Shake it around and you will hear loose stuff in the top tank, so try to shake them out.

Re the best radiator, I haven't had good luck with the original round-tops. They always seem to split a seam, both in my 69 Dart and 65 Newport. I was told that is common by radiator guys. You can put a 22" radiator in if you drill holes in the bracket or firewall. My 64 has an unknown one installed, probably even non-Mopar. I wouldn't worry about enlarging the frame opening, since I expect plenty of air still flows in.