Hi brrmm,
I've been trying to find stuff out for you, and I think it'll work out.
For the 85 323i's there are 2 different gearboxes, called the Getrag 245 and the Getrag 260, both 5speed gearboxes.
The Getrag 260 was also used in the 325i, so that should bolt up to your 2.5l engine.
Try this website:
http://www.realoem.com/bmw/select.doSee which parts belong to which car, and you can figure out if it'll work.
If you need more help, I'll contact the real e30 specialist for you!