I did read a thread here maybe a year ago, that said even though they are matched at the factory, our members have successfully used gearcase halves off different motors .
Hope those members come forward and comment on their experience. Can’t remember what year /models were mentioned.

On the Big Twin/RD split gearcases, the front bearing race and the rear bearing keep everything lined up automatically in the sideways direction but what can be a concern is the machined depth of the cavity for the front tapered bearing, which is also the thrust bearing taking the forward load of the prop.

The prop shaft should be driven forward while tightening the screws, as this will help seat the front bearing squarely in the housing and as long as there is a small amount of end play and the prop shaft turns freely, you should be OK. Doing a dry run first without the pinion or gears installed will let you know if the shaft is free or not.

As stated, the gear housings were made with two separate pieces from two separate molds or dies and then mated and machined together. The housing wasn’t cast in one piece and then cut in half. What you’re looking at is the amount of tolerance OMMC allowed between the two dies. The quality control and machining tolerances must have been quite tight back then as I have done a few of these swaps with no problems although in a perfect world, a complete gear housing swap would be ideal.