Heater core isn’t fitting in Plymouth Duster

O I found year one has a Deal on A body engine mounts 4 169.00 rear munt 59.00 my old stuff was 295.00 4 lft mount and 200.00 for pass side. I bought. and the were were old . do not trust any body but retail people.
o point is I have 2 383 67 notch back 33k and a 68 383 fastback. 101k am restoring.