[djpateman]

Matt is the expert, so I cannot add much except for the advice to consider how you want to do the task. The ring and pinion are a matched set, and the gaskets between the intermediate plate and the main case set the pinion depth. The depth was determined at the factory and adjusted by the gasket pack. Provided you replace the pack with the same thickness you should not have to go through the process of adjustment again; it is tricky.
My method of exchange would be to slip all the parts off of the 911 pinion shaft, and on to the 901 pinion shaft, and then use the 911 mainshaft as it is. Now you probably will want to go through everything and use the best components, and fix any problems.