I probably bought wrong 8 3/4

Sounds like an A body 8 3/4 to me. FYI, ALL A body 8 3/4 rears were 4" bolt pattern. You should be able to bolt this unit right in, and shorten the driveshaft to size. All A body 8 3/4 rears are the same width, both perch to perch and flange to flange. Only the brake distribution block is in a different location after 1970, where it was moved from almost on the radius of the center to a position more on the flat of the tube. You got the right stuff!!! Geof