Right - here goes !
I have a 1988 325i se and used a 1988 E34 535i engine, gearbox, ecu and engine wiring loom.
I used the E30.de engine mount brackets from Tino and I have used position 3 (furthest back).
I did take ALL the soundproofing off the bulkhead - to save weight and make room (I have read of gearbox bell housing bolts being buried in it)! I cheated a little here too as I cut the top bolt lug off the gearbox so when I have to take the box out again when my clutch explodes it will save hassle and knuckle skin! I used (new) standard E30 325i mounting rubbers. M30 gearbox fits in E30 tunnel but I had to extend the reverse light switch wires by about 6" and wished I plugged it in before the exhaust was on!
I had to knock the bulkhead in about 1.5"-2" just behind where the cylinder head is. I got my engine in position 2 first without the gearbox on, saw where I needed to hit it with a hammer, tilted the engine forward and gave it a good whacking! It's just next to the battery tray.
You need to use the E30 cross member, E30 or E34 rubber mounts will work here. It works for any engine position as the bolts have square heads that can slide in a slot to suit the position (almost as if BMW catered for engine conversion)!
My main wiring problem is that the heater didn't work when I got it running. Just needed the black and green wire on the E30 part of the big plug (C101) cutting and connecting to the smallest terminal on the starter motor. There are two wires in the plug that need swapping (can't remember which, there is a thread on the Zone with the info or I can have a look at my car another time if you get stuck) that enables the 'swingometer' economy(

) guage to work. Engine should run just by plugging the two looms together though.
Apart from that I just moved my coil slightly so those wires reached, moved my earth point on the right hand suspension strut top so the earth wires reached and turned the ecu round 180 degrees in the standard place - the wires do just reach with carefull routing!
Hope this helps!