66 Dodge Dart GT- Deep Sixing Slant 6 and Automatic for late model 5.7 or 5.9

iif that magnum engine you got is a 5.9 it had a 46re not a 44re trans behind it. 44 will bolt up but wont last especially if you really use the throttle. ive swapped 5.9s into a few jeeps was going to put one into my 65 dart before I changed direction and decided to go with a hemi