You might have to change the engine mounts, the tranny, the driveshaft, the rear diff, not to mention the engine and transmission ECU and fool any systems into believing that it's the original Tranny ECU if it's connected to anything. The tranny mounts might be different. From tranny to engine mounts might be different also that is why you'd have to replace the tranny or find/make an adaptor plate to mate the engine to the tranny if they are not the same. Any oil and cooling systems might need to be swapped and you should look into how your speedo will be affected. You'll have to find out which system both the 3- and 7-series use for the speed reading.
If you can find/make an adaptor plate for the 733's tranny, then it's just a straight engine swap, ECU swap, etc, and the running gear can probably be left in place.
I see you already posted over at the E23 Roadfly forum's so there's lots of experience chaps over there who'll be able to provide more details. It might not be as complicated as I mentioned
