The mainshaft comes out through the top of the box, not out the rear. To do that, the input shaft has to be out of the box to provide enough clearance. To get the input shaft out, the layshaft has to drop down. ....and the shin bone's connected to the knee bone, and the knee bone's connected to the..... Oh, sorry.

If you don't want to disturb the layshaft bearings, insert a dummy countershaft as you remove the original (see my post below).

For bearings, take the old ones to a good bearing supplier. They'll match them up fine.
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Steve.