I changed my 72 Challenger from factory 3 speed on floor to 4 speed.
Make sure the trans is for a E body ( I think E & B used same one, long tail stock)
once you get the trans should bolt right in.
Just make sure the input spline is the same and overall lenght the same. The 4 speed I got was out of a 440 6 PAK Cuda and I got everything since they changed it over to auto. Used the same drive shaft, but had to get 18 spline clutch since it was a Hemi 4 speed ( needed clutch anyway)