If you wanted to use the BMW diff you would need to make substantial mountings for it, similar to the Sierra set up. With the hubs you could have made a bearing carrier to take the BMW bearings which could be bolted to the upright like the Sierra hub or use a shaft which is BMW at the diff and Sierra at the hub.

You're going to need custom driveshafts whichever one you choose 'cos the track of the BMW and Sierra are different.

Cheers

Chris