Back in the "old days", stock eliminator rules required the original rear axle assy. The 8 3/4 did not become available in the "A" bodies, untill 1966. So they needed the gears for the 7 1/4 to be competitive. I ran a 65 Valiant with a 170 slant in Z/SA during the mid '70's, but I chose a 3.91 SG with a 8x22x13 tire. I do have a 7 1/4 LBP 5.13 SG rear in my shop.