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

It's your money and your car, so ultimately it's your decision. If it were mine I would have it inspected and rebuilt by an experienced Sure Grip person/shop. If this is original it's 40 plus years old and probably needs a rebuild. Cost of new clutch pack is $75 from Mancini Racing: http://chucker54.stores.yahoo.net/difclutkit.html