71-74 8.75 b body rear

That list above is TRACK width which has nothing to do with actual rear end length.

All B body measurements

Here's drum to drum
'65-'67= 59 1/2"
'68-'70= 60 1/8"
'71-'74= 63"

Housing width
'65-'67= 54 1/4"
'68-'70= 54 15/16"
'71-'74= 57 7/8"

Perches
62-'70= 44"
'71-'74= 47.3

It's tough enough trying to use a 68-70 B rear and the required wheel offset, it would be crazy offset with a 71-74 rear.