There's a very nice 8 3/4 axle in the AMT '71 Charger, should you choose that avenue, JC. Photo by Casey Littman.
The only negative is the AMT axle has the spring windup bumper above the pinion. That bumper appears to be absent on your posted photo.
My educated guess is the following. Road racers would prefer an axle with a removable center section, for the ease of axle ratio changes. Several pre-set sections could be kept on hand to easily tailor the axle ratio to suit a particular track. The 8 3/4 (and the 9 inch Ford) would suit that method.
Just my humble opinion, JC. I enjoy following along here and on the racing forum .