I looked into this as I have a B4.1 & wanted to go down the X6^ route.
The diff outdrives & bearings on the B4.1 are metric whereas on the B4 they are imperial therefore you need the old B4 outdrives & bearings to fit the X6 gearbox...
It begs the question why did AE change these to metric & leave the rest imperial
& why don't X Factory address this as they currently (I believe) make a conversion for a buggy you can't buy anymore. I know you can buy the outdrives & bearings but it bumps up the price by about £25...