the hardest part will be unbolting the gearbox from the engine. Most VN's have this heavy cast bell housing cover/brace and you need to remove this to disconnect the torque converter from the flex plate (I would try to pull the torque converter out with the gearbox). To get the cast bell housing cover/brace off your going to have to take off the engine pipes between the manifolds and the cat, unbolt the steering rack from the cross member and the steering shaft and pull it back and youer going to also need to remove the starter motor (make sure you disconnect the battery first or there will be sparks).
Depending on how sagged your engien mounts are you may have to jack up the engine to get the bell housing cover/brace out between the sump and cross member.
It will also be easier to turn the engine to remove the torque converter to flex plate bolts if you take out the spark plugs.
as for dropping the gearbox get a trolley jack adaper so it doesn't fall off or rig up a tie down strap and a bit of flat steel on your jack foot.
I'd probably drain the gearbox as when the filler tube falls out it will go all over the place, so drain the box and replace the sump. Undo the shift linkage, speedo wires, tail shaft and support it on a jack and tie it to the jack then remove it's crossmember and lower it down and backwards making sure the torque converter doesn't come off the input shaft. you may want to rig up a device to hold it in place, a bit of metal and a bolt through one of the holes on the front of the bellhousing for example?