rearend swap

I think you will find the difference in width is because the brakes on the 8 1/4" are 2.5" wide and the 7 1/4" brakes are 1.75" wide. Try measuring from the mounting surface of the drum to the other drum mounting surface.

FWIW, I swapped an 8 1/4" into my 68 Barracuda in place of the 7 1/4" and it was a straight swap. I also have ralley wheels and I had the drive shaft shorted 1 1/2".