B-body housing specs:

'65-'67: 54 1/4" with 44" perch on center

'68-'70: 54 15/16" with 44" perch on center

'71-'74: 57 7/8" with 47.3" perch on center

***I make a 1 1/4" offset spring hanger/shackle kit that can be used to mount a '68-'70 B-body rear into a '71-'74 B-body.