From my understanding usually the Ford 9"pinion is offset to the passenger side 1/2" and the axles aren't going to be the same length because of this.
I believe all first Gens put the motor/trans to the passenger side 1/2" to offset the driver/passenger weight.
Id call or look it up to be sure, I'm a rookie at this myself. You don't want to have to pay to do your 9" twice
