Tell us what parts you are replacing SPECIFICALLY, please.
The hole in the back of the 1972 crank is bigger, so if you're replacing just the engine ONLY, then the only thing you'd need to change is to get a spacer ring for the back of the crank so it will fit the early transmission.
The "old" converter you've been using would continue to fit the transmission input shaft, if you continue to use the "old" tranny. BUT, if you change to a 1972 transmission, you'll have to get a late model (1972-up) toqrue converter, in which case, you will no longer need the spacer ring for the crank-to-converter snout. The '72-up tranny has a bigger input shaft with more splines to match the splines in the late converter.
Clear as mud???? sorry...
