just take the diff from a 320iA - 4,10 (US-version). that truely works with your cardan shaft, mounting casing and your drive shafts... i've a 3,45 diff from the 320iC euro-edition in my M50 and the acceleration is quite cool. 4,27 or 4,10 might be to hard for your 5speed Getrac SD250.
(I'm searching for a cheap 6 speed gearbox from a M3 3.2L and that cardan shaft...so you've Vmax too in connection with a short diff

)
greets
Looky