4 speed ka-booom !

daliant said
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.

Yes I did kind of spot glue the gasket on the case. I'm not absolutely positive (mostly because I don't have one) but everything I've heard is you can't set the counter shaft before the main shaft in an overdrive. As I recall correctly thats what I was told when I called brewers Proformance.