Terry1,
You might try this question in England:
http://www.cobraclub.com/
One difficulty is that the XJ is wider, leading to some potential problems getting everything to fit under the fenders.
Lots of US-made kits use the Jag rear, but even if the XJ center section is used, the axles are normally made significantly shorter than the XJ axle length.
If you do a little searching, you can probably find the track widths for Cobras and XJ series cars and compare for yourself.
Tom