9 bolt is jsut a touch bigger diameter ring gear than the 10 bolt. I managed to break the spyder gears in my 9 bolt on a mid 13 second car but my 10 bolt now has held my nitroused 383 into the 1.4x 60 foot range and mid high 10's. So its hit or miss

Gm should have given these cars much stronger rears... some sort of 8.5" 10 bolt variant would have been nice

 

Maybe I'm missing something, but shouldn't the OP just take the ring and pinion out of his donor rear end and install them in his current rear end? Keep the carrier and axles as-is and just swap the gears.

 

if he has a 26 spline 3rd gen rear coming the 4th gen stuff wont work as its 28spline. 28 spline is nicer to have since they are stronger

3rd gen rear is shorter 1.5 inches per side. Actually its more like 1.75" depending on rear.