Step by step, article about fitting a BMW gearbox into a Volvo
I'v recently fitted one in my car, and here it's what you have to do

You need some hardware like :

* BMW 6 or 7-series Getrag gearbox(with separate gearbox/bellhousing)(I pulled mine out of a 732i late 70')
* The front part of the driveshaft from the same car(You only need the union that fits to the gearbox)
* Get a BMW 3-series clutch(215mm), M3 if you can offord it, or order a sinterclutch(as i did)
* M45 bellhousing(funny if you haven't got one already)
* Flywheelbearing from BMW(needle type)
* A rectangular pice of 12mm thick steelplate

on this picture you can see the BMW getrag gearbox , adapterplate and the Volvo bellhousing
The sinter I ordered fits the inputshaft's splines on the BMW gearbox, and I could use orginal Volvo pressureplate(from a 240 Turbo) and flywheel.
The 12mm adapter plate has to be machined so it won't touch the cover on the gearbox.
A friend of mine did this with a oxyacetylene torch. I resued the 4 12mm stud bolts from the BMW bellhousing.(suppose those are strong enough)
The m45 bellhousing also need some work. You have to turn out some metal in the center.
The linkage, and the plate where the gear lever is placed, have to be shortened about 2cm.