only reasonable one is a 633csi diff out of a manual car.
another options are 524td 528i,and 535i.
prob with the last ones is its a pain to find one with a decent ratio since almost all of them were autos.
the best ratio available is the 3.25 from the csi. but you can prolly swap the ring and pinion for a 4.10 or 3.93 or even the 4.27 if your not crazy about high speeds.
or if you are baller you can buy a 188mm spool for 600 bucks from metrix motorsports.