Heater core diffs

May not help much, but I replaced the HC in my 73 Sport project with one from Classic Industries. Did not fit, the tubes were just enough out of position. I returned it and bought one from RockAuto, it also did not fit. Again, the tubes were off. I ended up carrying that one, with my old one, to the radiator shop where they bent and re-brazed the tubes so the new one matched the old one.
Guess what I'm saying is even one that's supposed to be correct may not work.