Back spacing on B body rear end in 73 Duster

'68-'70 B body rears are 54 15/16", '65-'67 should be 54 1/4". So like Crackedback said, about 5/16" shorter per side for the '65-'67 B rears.

Based on squeezing every inch of space out of my Duster's stock wheel tubs, I would say you're going to need to be between 5.5" and 5.75" of backspace on an 8" wide rim. And a little "too much" can be dealt with using a small spacer, so I'd be inclined to go with 5.75" unless your measurements show much different.