Understood. Depending upon the 455 you've built there are several ways to go. If you aren't talking hugely more than stock 455 and have something like a 3.08 already(Firebob's question), you may be able to just install an aftermarket posi unit. Obviously, swapping in a known rear end and making the necessary modifications to spring perches, driveshaft, u-joint, etc. is another viable direction. Lots of choices, that's why the questions.