[northNH]

This is a “M40/41 Transmission Rebuild 101” question…
Correct procedure described in factory manuals, Haynes, etc, etc.

You need a wooden dowel of identical diameter (~3/4”) to the layshaft gear cluster center rod, just long enough to fit inside the cluster without protruding. This will allow the needle bearings to remain in place during installation.

Lay the cluster with it’s shortened dowel shaft and bearings in the bottom of the case and completely assemble the mainshaft cluster above it.

Turn the case upside down allowing the layshaft cluster to engage the mainshaft cluster and use the steel layshaft rod to drive out and ultimately replace the wooden dowel.

Nothing tricky here, just good clean fun.
Good luck.