Hi Danny,
I have fitted taper roller bearings to my minilathe. I avoided most of the problems with the internal spacers by making up two washers 1.5 mm thick so that the bearings are effectively the same thickness as the standard bearings.
Once the bearings have been fitted on the spindle you can try the existing rear spacer for fit by assembling the spindle including spacer and bull gear and then tightening up the spindle nut. Make the nut really tight to ensure that the bearings are well seated.
The spacer will be too long so decide how much to remove in order to get the bull gear lined up with the tumbler reverse gears. Now undo the spindle nut, remove the bull gear and spacer. Make a tempory spacer from a piece of 1.1/4 polyprop waste pipe. This is available from plumbing stores. The tempory spacer can be just sawn to approximately the right length. Now reassemble the spindle with the tempory spacer, bull gear etc. Now you can use the lathe to trim the old spacer to the right length. The plastic machines easily.
Finally dismantle the spindle again put on the trimmed old spacer and reassemble.
The tempory spacer works because there is little strain on the lathe whilst machining the old spacer.
There is a complete description of this modification on my website:
Headstock upgrade - mikesworkshop
The taper roller bearing are certainly a worthwhile mod.
Mike