Saw this company statement on an eBay Xjs item. Sounds like they have experience of carrying out Manual Conversions and worth a telephone call at the very least:-

“We have many other performance modification especially for the V12’s such as manual conversion, throttle bodies and much much more! Please see our other listings. Over 30 Jaguars in stock for parts. We are specialists in XJ’s!
For all enquiries please call or email the Simplyperformance team directly as it will result in a faster and more efficient reply. Jon@simplyperformance.com – 07796 002321 – Please do not message through eBay. If you do please leave a contact telephone number.