charlandk Kevin, The way I read this question is are you running an open carrier read or looking at a posi one?
If this is it, Posi over open, reasons why are many. Depending on which carrier you get, it's about putting power to the rear tires and movement. Posi in a straight line both tires work, open rearend in a straight line the r/r tire works only. There are 4-5 kinds of posi's. clutches, air, worn gears, mini lockers, mini spools. The question is what kind of HP is the motor going to have, what are the trans ratio's in the trans you are looking to run, what kind and size tires are you going with?? IMO a posi is needed in any cobra with 300hp or more for control of the car.
Kevin if you have the info on your car on another thread, it would be nice to know. You have not really said what you plan to use the car for?? Cruising, Shows, autocross, road racing, stress release, or chick magnet???
The other question is without knowing the HP and more important thing, TORQUE you are leaving more questions that need answers. A stock rebuilt Jag rearend is good for about 450HP and Torque. Some other rearends will take more or less power limits before breaking. If you normal drive the car, no additional improvements need to be made. If you are going for heavy abuse you need to look at better output shaft assemblies that are good for 750-1,000HP. Chromemoly stub shafts in the carrier. Better sidegears in the carrier and even blueprinting the unit itself. OVER BUILT your car.
This way it will take some extra abuse and not break down on the side of the road from a drive train failure. The rearend in my car is all heavy duty parts. Chromemoly shafts, 30 spline locker, heavyduty end supports, gridle for the carrier caps. HP rating is in the 650 range. Been racing this setup for 12 years and no failures. The first year I broke a stub shaft bearing, have had no failures since then. Have gone from 448ft of torque to over 600ft of torque, same rearend and tires. Time will tell about failures of these parts. Rick L.