After spending some time searching for front end gears (just in case) for my L2500, I found that most (if not all) Kubota drive pinions and ring gears are only sold in sets. Same is true for your B2400 ($323).
I've set up probably a dozen differentials in my past, and it's never a Good Idea to mix and match from different gear sets.
Getting the proper setup can be a bit tedious, especially if the bearing is pressed onto the drive pinion. Can't tell from the Parts Diagram. [If the bearing is pressed on, you need to make a "setup bearing" by taking a new bearing and 'ruining' it, by grinding some out of the ID, so that it will slip on and off the drive pinion to allow you to change collars or shims.]
As far as determining what "collar" you need, unless you have the special tools, it's done by trial and error.
Not every drive pinion is the same, due to machining variation. Some manufacturers mark the drive pinions in a way that allows you to calculate just how similar or different it is from what you had.
Very simply, the way you set up a differential is to make an educated guess and install the shims (or collar) that's close, and then use marking paint to check the gear mesh. If it's not perfect, you disassemble, use a different collar/shim, and try again.
Note that you have the same issue with the bevel gear that the pinion engages. When you change out a gear set, you're (usually) starting from scratch. Sometimes, you get lucky and find a donor gear set that has the same setup as yours. Generally, then, the gear mesh will be correct when you check it.
Kind of daunting to think about stocking all those collars, if you only intend to build one differential, because in the end, there's only one that provides the perfect setup.
I were you, before diving in, I would first exhaust all your options to buy a complete good rear from a parts tractor, still with the factory setup.
To determine my options when I was anticipating this challenge, I surveyed the Parts Catalogs to find all the Kubota tractor models that used the same gear set (yours is 6C040-97960), and use that info to widen your search for a used front axle to all the Kubota tractor models that used that gear set. Often, you can transplant all the parts from the donor axle (including collars/shims) and it will be very close to a good setup, but not always. If you're REALLY lucky, you might find a wrecked tractor with a broken housing that you can buy for cheap. I'm never averse to paying double scrap price for something like that, and scrap price is low-LOW right now.
Maybe all this stuff is not quite as critical in a differential that is not spinning at a high speed, generating all that friction...I dunno about that.
-Paul
