Mopar 8.75 posi. wont work. Why are there 2 rows of spline?

If you have the rebuild-able Spicer clutch pack you can use Dana 44 spider gears. They have a solid spline (not split). No need to rebuild the rear if everything else looks good. You can reuse your axles then. Although if you plan on drag racing or hot rodding your car, you may have problems with axle strength in future.