Although not of immediate interest to many road-runners/street-burners, mainly accorded to the racing scene, it seems to be a perplexing problem to a very large number of folk around the world, and has burned up plenty of telephone time.
Assembling the engine to gearbox (transmission) is explained and simplified in this article. Contrary to popular mis-guidance, all BBUs will fit all four-syncro gearboxes. It was only the three-syncro boxes that varied in casting dimensions causing hassles where 1275s were applied to 850/998 gearbox casings. The longer throw and bigger crank and rod assembly dimensions fouled the sculpted areas in the gearbox case,