I'm currently in the process of converting my 1997 323i from auto to manual using a 1991 318i as a donor car for parts. There is some misinformation on the internet so this article is mainly about correcting that, which mainly comes down to parts compatibility.
The main tool which is invaluable is realoem. I donated.
The internet says that all Getrag G250s are the same, so putting a 4 cyl GB into a 6 cyl car is equivalent to the factory box. This is incorrect, the 4 cyl GB has a suffix on the part number, mine was TBDH, whereas the 6 cyl GB has TAJT. The internals can be compared on realoem (image below, 4 cyl GB on left) which reveals that 1st and 2nd gear differ quite significantly, there are additional parts in the 6 cyl gearbox, and different part numbers for the gears themselves. I assume that the strength of the actual gear isn't different, but this could explain people blowing up synchros and related parts. Would love to hear from someone who knows about gearboxes!
Having looked into this more, the 6 cylinder gearbox has discrete synchros on 1st and 2nd gear. First and second gear themselves are different part numbers, assumedly to interface with the different synchros. My opinion is that the strength of the gearboxes won't be different, but the 6 cyl gearbox will take 'spirited driving' better, 1st and 2nd synchros will last longer and behave better in general.
6 cylinder automatic drive shaft on the left, 4 cylinder manual drive shaft on the right. The front and rear are different lengths, the front depends on the gearbox, the rear depends on the diff. Apparently the 4 cylinder front will fit behind a 6 cylinder, it needs to have the spline inserted a further 20mm (see shift linkages info below). I'm yet to confirm this but the measurements lead me to believe it will work.
Another thing, from what I've read, and comparing parts on realoem, it seems safe to remove the weighted ring at the front of the 4 cylinder driveshaft when it's used behind a 6 cylinder. This ring is a vibration damper and is only specified on 4 cylinder cars. The 6 cylinder driveshaft isn't just the 4 cylinder drive shaft without the damper though (they're different lengths at least), so do this at your own risk.
The 4 cylinder shift linkages need to be shortened but by how much seems to be a guarded secret, many people guess, some seem to measure. Turns out it's written on the shifting arms! So the shifting arm and linkage need to be exactly 20mm shorter. I also think it's safe to assume that the difference in lengths between the front section of the drive shafts is also 20mm.
