I noticed spools on Ebay referring to 30 spline and 35 spline axles. Are each of these spline counts specific to any specific application? Would an A body most likely use the 30 spline axles?

(1) probably but not sure what app is what (2) yes

As far as I know the 8 3/4 has one spline count, period.

No one ever wonder if the SG would fit based on spline count.

Sounds like a non-8 3/4 posting in that ad, but it is Ebay, not like it's the fount of accuracy.

I went through this same dilemma when I bought my TruTrac earlier this year. They are 30 spline.

Stock axles are 30 spline. You can get aftermarket axles in 35 spline.

I have one of those spools. I have not tried to install it yet, but it looks very nice.

30 spline 8.75 are the same as 30 spline dana 60...

All the stock Mopar 8 3/4 rear ends came with 30 spline axles, all of them When Mopar started using the Dana 60 rear ends in the H.D. cars options they came in several different spline counts with 35 spline being offered in 1970-1971. The 8 3/4 aftermarket spools are offered with 30 and 35 splines so you can use either axle spline you want, the 35 spline axles are a bunch bigger and better for street and drag racing a car that has good HP and hook with a 8 3/4. Bottom line is the 8 3/4 are a decent rear end for most applications, they where never designed nor intended to be used in high HP cars for drag racing If you are planning on racing a car that weighs above 3000 lbs with the driver in the high tens or quicker in the 1/4 mile and you need to build a rear end for the car you may want to think about using a Dana 60 to start with