I agree with lively on this point. Dollar for dollar a 9 inch is the way to go especially if your talking relatively stock configuration. A 12 bolt traditionally takes less horsepower to turn but that's about where the advantages end. 9" is stronger, easier to work on ( I'd much rather have a third member on the bench setting up gears then rolling around on the floor or in the pits. I think also if your at the track and needed parts you'd be much better off with a 9". You also don't have to add c- clip eliminators like on a 12 bolt.
I agree with Ed that you really either need a nodular iron or an aluminum thru-bolt case, and an aluminum pinion support. Somtimes the ring gear must be chamfered so it clears the pinion pilot bearing area.
