There are two (at least) different sizes of bearings, it seems to be that where flanged diffs and Lobro joints are used it is always the larger bearing, however we have seen CV joint driveshafts with both sizes.

In the picture the top drivershaft/hub is the type with Lobro joints and flanged diff, the bottom one is the CV joint type.
On the Lobro joint one the surface on which the inner bearing sits is part of the hub, on the CV joint type the inner race sits on the drive shaft,
To check which bearing your donor parts need simply measure the diameter of the part where the bearing inner sits.
The smaller bearing is 35mm (60 x 35 x 37), and the larger is 41mm (68 x 41 x 37)
Some guys seem to be having problems with finding the right bearings, also there have been some unexplained failures (maybe cheap Chinese bearings) and we have had some delays and problems with machining centres for our uprights.
We have now re-designed the rear uprights to accommodate more 'modern' bearings, this way we can produce one upright and supply bearings to suit any of the donor parts.
The new bearings are 'bearing units' that is rather than being two separate inners, two outers and two bearings, plus the two oil seals, they are one unit, pre-greased, preloaded with integral seals. So no more packing bearings with grease and no more struggling to fit the oil seals which inevitably end up damaged.
Unfortunately we could not find an exact bearing to suit both sizes of driveshaft, so there is some modification required to the larger (Lobro joint) donor parts, this is a relatively straightforward mod that can be done by anyone with (competent) access to a lathe, or any machine shop, or we can get the mod done for a minimal charge (free to purchasers of starter kits).
The diagram below should be enough info to get the mod done yourself.

UPDATE Another option is to leave the 40mm length 'as is' and use a 4mm spacer between the hub and the bearing, this is probably better than machining the length.
Basically the 41mm diameter is reduced to 39mm, and the length of the bearing surface reduced from 40mm to 36.5mm. Obviously make sure the fillets or chamfers of the originals are duplicated to avoid sharp corners which 'attract' stress.
If you are planning on using the Golf calipers and the smaller (and lighter disks) then get the outside diameter of the hub machined down to 135mm at the same time.
The new uprights are in production now and will be supplied with all future kits, should you wish to 'upgrade' your old uprights to the new ones this is a straight swap-over, (with machining of hubs if needed), contact Ann if you wish to do this, some guys may qualify for a free upgrade

Brad