Hmm, my memory isn't exactly clear, but I don't think they dropped much more than that.
Not sure I follow you about the shims, but the usual way is to remove the rear bearing with the shaft still inside the housing. You tap the shaft to the rear just enough to get a bearing separator under the circlip, then either use a puller to get the bearing off the shaft, or put shims under the circlip and then tap the shaft through the bearing (repeat as necessary to work the bearing out). Page 7 at
https://vintagetriumphregister.org/maintain/TransRebuild/TRTrans01.pdf shows the process using what I believe is a cheap separator from Harbor Freight with some simple mods to increase it's reach (threaded rod & long bolts from the hardware store).
If you really can't get the cluster gears to drop, then you may be able to get a pry between the back gear and the bearing, to move the shaft forward a bit and then drive it back as above to work the bearing loose from the case. Be careful though, and stop if the gear teeth get tangled. You don't want to break the gears.
Yet another approach might be to use a small grinder (eg Dremel) and grind the bearing apart. Cut the metal bands that hold the balls in position, then grind notches into one or both races so you can remove the balls one at a time. Very tedious and time consuming, but should be possible. Be careful not to cut the case or shaft (although one or two small nicks won't hurt anything). (I've done other bearings this way, never had to use it on a TR though.)