Original Rear end type on my 70 Duster

Most 1970 and newer v8 cars (notice I said “most”) had an 8-3/4” rear. 3:23 was a common gear ratio.
7-1/4” was another option but I believe that was /6 only by 1970.

I too agree 8-1/4” came out in 1973 to replace the 8-3/4” rear. Not sure if they ever made a small bolt pattern 8-1/4”