With a T20 it's easy to suspend the weight of the transmission by the clutch and use that weight to assist in the removal, but with the black box would be a little more of a trick to do that. If there's any way to suspend at least part of the weight by supporting under the clutch, it may help.
I've never seen what's in one of those mysterious transmission boxes: any chance of posting some pictures? I see you've got the bolts out of the cover......
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far