My Datsun 1200 parts book, issued December 1974, shows only one number for the main shaft & it is the first one on your list. Does your book indicate that the new shaft is interchangeable with the earlier one?

The transmission assembly is probably interchangeable conditionally. That means that you would probably need to change some other parts, like perhaps the earlier rear oil seal [if applicable] & you might need to use the earlier clutch arm or delete some smog switch. We interchange these gearboxes all the time. It takes only one tiny difference & they declare that some parts are not interchangeable.

All rolling element bearings are good, but their load carrying ability depends on their size. The 56A gearbox continues to use ball bearings to support the input shaft, the rear of the main shaft & the rear of the cluster, so they are not an inferior design pre-se, but the closed end roller bearing for the cluster saved quite a bit of machining, several separately made parts, custom fitting of the bearing with shims & additional assembly functions.
While it saved all this, it was obviously also quite adequate in the strength department too. There is nothing at all wrong with the newer design, but I personally prefer the ball bearing from a design/ engineering standpoint. In reality, the ball bearing is probably three times stronger that it ever needs to be while the roller bearing is probably only twice as strong as it would ever need to be. Like killing ants with a three pound hammer, or a two pound one I suppose.

Are you able to confirm, or otherwise, the interchangeability of the two versions of the cluster?