First thing to note is that the automatic transmissions from the bigger 5 and 7 series are well, BIGGER. This means that you have to heavily modify (ummmm hammer?) your transmission tunnel to get one of these damn things to fit.
An E34/ E38 540i or 740i with a 5-speed automatic transmission (Called the ZF 5HP30) is an ideal choice. The 5HP30 was used in the E38 and E39 for a long time, but these transmissions had issues in early E34 cars. It wasn't uncommon to get an auto e34 that wouldn't go into reverse. So if possible, get these transmissions with a manufacture date after 97.
With these automatic transmission a custom driveshaft is required.
This is the cheapest route. Many e34 530i cars were 5 speed manuals, so these can be found from junkyards easily (however ideally you should just find one out of a running car). These are cheaper than the 6 speeds, smaller + lighter, and do not require the use of a custom driveshaft. The e30 m3 driveshaft works with no modification. The front half of an e28 535i can also be used with the back half of an e30.
Cliff notes: You use a stock, large guibo driveshaft (late 325e, e30 M3, or e36 328/M3 with 4 bolt rear).
As far as the mount goes. No modification is needed to the stock e30 manual transmission cross-member.
Note: 5 speed weights about 85lbs wet
For the clutch, you must use a 240mm flywheel only. However, any E36 or E39 B28 clutch kit will work.
This was the lela car choice. Although heavier, the 6 speed does have its appeal on the highway. It did fit in the transmission tunnel with little issues (you need to massage the trans tunnel). However, the shifter did require a lot more work. The shifter has to be bent and shortened to clear the much bigger guibo.
As far as driveshafts you have two options. Shorten an e30 m3 driveshaft by 7cm or get the custom driveshaft completely brand new from us.
Front piece needs to be 20.5", back half must be 32.5"
Note: 6 speed weights about 110lbs dry
For a 6 speed clutch setup you may use either a 240mm or 265mm flywheel. However, you are limited to that respective V8 clutch kit only. (now is a great time to get that lightweight flywheel!)
For the mount you actually need to extend the factory cross-member as shown below.
We offer 6 speed extension brackets pre-cut. All that is needed is for them to be welded up to your factory brace. (below are early prototype pictures, later pictures are of actual product).
How they should look when welded up:
Extension brackets we offer: