I would give Reider Racing a call. They live differentials for all sorts of racing applications. They may be able to help you source a set. I have one of their posi-locking gear sets in my Cobra.

www.reiderracing.com

Going to a higher ratio rear end is interesting... Would probably make first gear totally worthless.

You will also probably have to change the driven gear (if there is one and a replacement can be found) for the speedometer, as your mileage readings would be lower than actual. This could also effect the deployment of the rear wind wing? And who knows what else...?