idk. Although in my case, I wasn't able to respline my axles because the splines were higher than the rest of the axle, thus you may need brand new rear axles regardless. Did you get that 8 3/4 open from the guy?
For mine I did a write up on how I got my 8 3/4 in my 66 barracuda. Only this pertains to getting the rear housing cut for wheels with a 4.25 backspace, your measurements may be different. I'm assuming your rear end is out of a B Body.
The rear end is entirely new, to the car, it's an 8.75 out of a 68 Charger that was narrowed to 55 7/8 inch wide outside axle to outside axle end. With a 1/4 inch thick brake rotor that I bought from Dr. Diff the total width is 56 3/8. One side is narrower than the other in relation to the center as the body is off from the springs nearly 3/8 of an inch to the driver's side. You may have to measure to see if yours is different.
What you should do is get the car exactly where you want it ride height wise, then put the mounted wheels and tires exactly where you want them in the wheel wells and measure from back to back. I had to do a lot of measuring before we found out that the springs were off parallel in relation to the body. Which is why the driver's side is so tight and we have massive amounts of room on the passengers side (figuratively speaking).
