My recollection of the key points is
1. V6 (and I4) petrol engines use GM small block bolt pattern for the bell housing. BMW diseasal uses BMW bolt pattern.
2. Petrol and diesel engines have different torque characteristics therefore the torque converter is chosen to suit the engine.
3. AR35 is stronger than AR25 but has same gear ratios.
4. If the new gearbox came from a petrol engine then the bellhousing and torque converter need to be transfered from the old box to the new gearbox before fitting.
5. The gearbox must be stood on end before the bellhousing is removed or else the oil pump, and very important little things, will jump out and forget where they came from
6. There is a plastic thrust washer in there somewhere that is easy to change while the bellhousing is off.........
7. Assembly is reverse of dissembly BUT as said above fit torque converter to gearbox before fitting to engine.