CAN'T FIND 1970 DART 340 HEATER CORE AND EVAPORATOR

Road Grabber said
I plan to look into a radiator shop (if they still exist) and see what the cost would be compared to new ones.
Trouble is you don’t know wall thickness and and corrosion levels for even a core that passes a test.

Agreed, but it is your only option to test one. I usually up the pressure to give myself a little margin. Better than blindly putting it back in.