Originally Posted by 93redfox
You may look into seeing if frpp even makes a 28 spline carbon clutch pack set. It may be that they only make it in 31. The only way I know of checking the number of splines is to take the diff cover off and look. You might get lucky and find a part number stamped into the axle flange but don't bank on it.
The splines you are talking about are axle splines.
The axles fit into the axle gears, then the clutches fit over the axle gears on another set of splines, which I don't think are any different regardless of 28 or 31 splined axles.
Here are the clutches I believe were used in Army_Stangs differential.
8.8 Carbon Clutches