The best thing to do is obtain the JDHT CD for the XJR, as all the electrical manuals and diagrams are there. If you can read the diagrams, it will tell you all you need to know.
I'm looking into doing an XJR engine swap into a Daimler DS420 at the moment and a friend of mine who is an electrical engineer has looked over the diagrams and said it shouldn't be that difficult.
One thing I would do if I was you is get a North American market ECU, not a UK/European market ECU because the North American cars do not have the fuel inhibit signal from the security module. Or perhaps XJR Engineer can reprogram the ECU for you?
I know this combination of engine and transmission has been put into other vehicles, but I've never seen a tutorial of how to do it.