It's more than likely the gearbox not coming apart because its not perfectly straight. The two should basically 'fall' apart. Try this:
Lift the engine and gearbox to about 12 inches off the ground (make sure the bellhousing bolts are in and tight!!).
Put a couple of blocks of wood (enough that the oil pan is off the ground and everything level) under the gearbox, front and back.
Lower the hoist so that the gearbox is resting on the wood, but the engine is hanging.
Now loosen the bellhousing bolts and try to seperate. It shoud be easy to do if the gearbox input shaft is straight (in alignment with the splines on the clutch plate)
If it does not seperate, lift and or lower the engine in very small increments until it does.
Tim