Easiest swap is definitely one entire housing and all its guts for another. I just did that exact thing with a 69 rear end into my 64.
As I understand it, the 12 bolt got a little wider from axle flange to axle flange in 1970, but not by much, only an inch or two, which some people consider a better look under the wheel wells anyway. Panhard bar mount should be the same 65-72, just confirm both rear ends are coil spring as previously stated.
