Merkur XR4Ti ??

[XR4BILL]

Jeff, the best option would be to fire off an email to Chris at MC2 Racing. He could tell you exactly what they do.
There is a fair bit of info at the MC2 site as well. Also do a search at the Merkur Forum.
Not sure about what you ask, as I installed a 7.5`Cossie Diff into my XR. Basically a straight swap with some spacers etc. I have 2 stock Xr diffs lying around that I was considering using. I was also thinking about the Supra Diff into the XR and the Cossie Diff into the Kit car.
I will be looking at Diffs later, as I begin the build in the new year.
cheers,
Bill.