Technical information provided by Dave Cirillo
WM300 gearbox input shaft mod:
When installing a WM300 gearbox on a 61-68 hydraulic bellhousing you will find that the input shaft is too short. If you can find a 61-68 NP420 gearbox you can remove its shaft and install it in your WM300 box as has been done on the box pictured below. The mount ears on the WM300 gearbox will fit the 318/360 bellhousings.
1. Remove the shift cover from both gearboxes.
2. Remove the 4 bolts from the front bearing retainer and throwout bearing guide tube and remove it from the input shaft.
3. Rotate the shafts until the gap on the syncro gear (white arrow in lower left corner of photo) is up so that the shafts can be pulled out.
4. Make sure the input bearing on the long shaft is good and install it in the WM300 box using a new gasket under the bearing retainer. Make sure that the oil return gallery in the retainer is lined up with the hole in the case.