replacing 3.73 12bolt gears

[man-a-fre]

I have a original 12 bolt 3.73 posi rear in my 72 blazer 4x4 that the gears are bad,a friend gave me a 76 4x4 12bolt 3.73 gear rear with an open carrier.Thought the gears would be an easy setup but to my suprise the pinion teeth part on the 76 was larger in diameter and the ring gear was thinner than my 72 originals so it was going to be alot tougher swap than i thought.What year gears are the same as what i have or is it that the posi verses open carrier is the difference.My gears are original with the gm markings as are the 76 chevy 3.73 4x4 gears i tried.