I have a genuine syncro box if you want one. I also have the tooling drawings etc.
The XJR15 is a very well handling car; it has the same suspension and similar tub to the Le Mans cars. The handling issue is because they jacked it up for road use. Wonder how your cars would handle if they jacked them up 3 or 4 inches? If its lowered back down it handles very well again. We sorted one out to run in the Group C races a few years back, even got a 4th overall at Spa as i recall. I have a mold for the XJR15 LM front splitter. If you can find pics of Bryan Wingfields XJR15 you will see how it looks, yes we changed the rear wing too and lost the silencer box and replaced with the race straight throughs.