Hi Wahoo,
You can use any width rear end that you want, but you might end up with some funny looking offset for the wheels, or some really big fender flairs......
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Norrowing the Jag is not a big deal. Do a search on Concourse West, they have everything that you need for the Jag as well as lots of info on what works best.
Good luck!
Jon
P.S. I saw your other post about the front end and I've got a friend with a complete E-type front end for sale. Send me an email if you want and I'll give you his phone number.