4 speed ka-booom !

j par said

ATTENTION !! -- I'm almost positive that you have to drop the cluster gear in the case to install an OVERDRIVE 4 speed and then pull it up into place after the main shaft is installed. I'm sure its because the overdrive gear (now occupying third gears spot) is to big to clear the cluster.

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No other way to do it, the mainshaft gears won't clear the cluster gears with the counter shaft in place.
Also one tip I've learned is to glue the tail shaft gasket to the tailshaft, one less thing to worry about when trying to load the main shaft into the housing and keeps you from tearing it.