Help Identify 8 1/4 differential?

My 8 1/4 spring perches match the existing 7 1/4 dimensions at 43.02" so that part works fine. Per a chart off moparts that makes the setup a 73 to 76 A-body unit. Dont know why the casting numbers dont work but I am going to assume it is an anomoly at this point. The width on the 8 1/4 unit is 1.8" narrower per the chart which could move the back plates .9" closer to the leaf springs. I know the brake drums are wider so I am hoping that the axle face to face is the same as the 7 1/4 unit so my 15" LBP rallys will fit without rubbing. It looks like this unit will work on the surface, just need to buy new durms, shoes and cylinders for the 8 1/4 before installing.
DR