$hit and run repair
Test fitted the radiator tonight. Its an aluminum unit of unknown origin- it was installed in the 66 Dart wagon donor car.
It mounts very close to the 4 blade fan due to the 1/2" spacer needed to clear the fan from the PS pulley & belt.
Tonight I noticed a previously unknown detail of the radiator support. The radiator mounting flanges are different on each side. Originally a 273 4speed car, the drivers side flange is 3/8" closer to the motor than the passenger side. The radiator mounts a bit crooked as its mounting flanges are symmetrical.
I dont weld aluminum, so I'm considering trimming the radiator flanges so as to mount it to the front of the radiator support on the drivers side to even it out a tiny bit.