I thought I'd put up some info on what I've learn't with Datsun gearbox thrust bearing carriers.
Alot of variables here as clutch covers vary in height and clutch fork pivot inside the bellhousing is adjustable.
Generally there is a large bearing for the 225mm (6cyl) clutch covers and a small bearing for 200mm (4cyl) clutch cover, both interchangeable on carriers.
In my experience there is five different sizes, measured from bottom to bearing base they are 39mm 34mm 29mm 27mm 25mm.
Of these five maybe the second 34mm is aftermarket? seems to be steel, while all the rest are nissan cast.
What I can say is on my tall heavy duty 57mm cover i use a 25mm carrier
On my custom low HD 42mm cover I use a 39mm carrier
Standard 6cyl covers appear to be 47mm tall, pretty confident 39mm is the standard fitment here, given i have about 10 of them!
feel free to add.