When I pulled my gearbox, I had trouble getting it apart the first inch, but after that, it pretty easily pulled apart, (I had a bear of a time getting it back in). I would think the splines on the imput shaft and clutch are gummed. It did take some umph to get it apart, if I remember.
Other than the obvious of a bolt or bracket still connected, or the box up against the chassis, dropping (tilting) the engine a touch may help.
Mark B or Larry B know far more than I, perhaps they or one of the many other wrenches on the board have some more thoughts. It was about 3 years ago that I pulled mine.
Mark J.