Which 8.25 do I have?

We are swapping out the front drum brakes and rear end on our 68 slant six cuda with the 4" bolt pattern for disc brakes out of a 73 Dart with the 4.5" pattern.
We swapped the front brake parts today and it is back on some 15" stock Coronet wheels.

Our next step is to swap in a 8.25 rear end I picked up a number of years ago that also has the 4.5" bolt pattern. I know I will need a set of shock plates and 3" u-bolts. We are going to drop in some new Mopar XHD springs.

Is there a way of knowing for sure which model of vehicle this rear end came out of?
I measured between the spring perches and it looks to be about 43" center hole to hole. The measurement between the backing plate at the outer lip is about 48".