I'm looking for a posi rear, 3.73 gears, and drum brakes. i hardly ever see any thirdgen rears, but i see a lot of older chevy rear ends. will any of the older rears (camaros, chevelles, etc.) bolt right in the same as the stock one, or is modification needed?

 

Only a third or fourth gen diff will be a direct bolt-in. These cars use torque arm rear suspension. Those other diffs have no way to attach the torque arm. Those other diffs are also too wide to fit under a third gen.