The 5 series BMW diff I have sitting on the shelf has a ring gear a good step smaller than that a Ford 7 1/2' one but the diff is a nice strong ZF 4 pinion diff.
Halfshafts are about same diameter as 2wd, and the CV joints are the 4x4 or Scorp smaller diameter CVs but with M10 bolts.
MOST BMW ratios are crazy 2,87 or maybe 3,08 for their larger powered cars which seems odd but then again their box ratios are often 4.2 or so for 1st gear so it works out for them, Combing a BMW box and Ford diff or BMW diff and ford box is going to be weider than weird.

I've seen some really far-sighted and brilliant , and dashingly handsome guy way out in the far north West of America using Toyota MkIII Supra 8" diffs with honkin' huge stub shaft diameter and easy to fit and find and he made some sweet adaptor plates to allow using either 2wd Cossie or 4x4 sized CVs.



Stock ratios are 3,9 for early, 3.73 for later manual box and 4,3 for autotragique slush box.