I'm doing the same thing on my 72' and am in the process of looking for a replacement radiator.
For a 1972 Plymouth Duster with a 340 (w/ air conditioning & automatic transmission), it should be part number 3574658 or 3574634. Make sure it has the transmission cooling ports at the bottom if it's an automatic.
If it's a manual transmission, I think it's part number 3574636.
My understanding is that all of these original radiators used a 22" wide core, and anything bigger on a 72' will require some modification (i.e. cutting, welding, drilling, etc.).