To be honest, you'd be better off staying with the rear that's in the truck now. It's a truck style 12 bolt, far stonger that the 10 bolt from a Chevelle. Parts are still available for it, including gear sets. With an axle shaft swap to the 30 spline "mid-'65 and later) you can also open up alot of availability of virtually any kind of Posi ar locking diff that yis available. You can also change to an easy truck 5 lug (5x5) bolt pattern using later axles in the existing housing. I thnk that the need shortened slightly (I forget the exact amount) to fit, as the later housing is slightly wider. This can compliment a spindle swap in the front for upgrade disk brakes using totally stock parts.
