Joined: Nov 2016
You have to run axles set up for your particular ring and pinion set. It's not just the number of splines, but the size of the axles that matters, as higher spline counts are physically larger. Your bearings need to be the right size as well. So bottom line? Either get the factory replacements for your factory parts, or else get a matching set from one end to the other, so it all works. To do that, you need to deal with the manufacturer and find out the specs.
To keep things simple, your best bet (with a factory axle) would be calling a reputable manufacturer like Strange and finding out what they recommend for your intended use, type of tires, and horsepower level. Any 9" is probably overkill unless you're running slicks with some serious torque. For street driven cars with a small block, 8" rear ends are often better, due to less parasitic power loss and lighter weight. Even if you soup it up pretty good, usually the extra power gets released in the form of tire smoke long before parts start breaking.
I smile a lot. It makes people wonder what I'm up to...