I have heard from a Jaguar Specialist that it is not that straightforward. That said, they did a conversion on an XJS to 5 speed manual and Supercharger at the same time. Some modification required to go into the XJS.
I have a 1990 3.6 xjs, and purely by chance I happened to glance up at the underside of the boot and there to my amazement was a little factory fitted sticker with my cats code on it! Hope yours is...