There is no need to remove the shifter in order to remove the top cover of the transmission.
Disconnect the parking brake linkage (one cotter pin).
Remove the 2 screws in the side of the cowl. ....Originally these did not have nuts on the inside, so there may or not have nuts inside.
Remove the cap screws holding the cover down.
Pry the rear of the cover up, and raise it as you slide the cover toward the rear. ....The bottom of the gearshift can clear everything if you raise the cover enough.
To help in re-assembly, glue the new gasket to the top of the transmission, but take care not to cover the vent (porous brass plug) with gasket goo. ....Let the gasket goo set up, so the gasket doesn't move.
Apply a smear of grease on the top of the new gasket and re-install the transmission cover. .....The grease helps seal, but allows the cover to be removed in the future without tearing (or needing) a new gasket.