It just rest on it's thrust bearings - no problem.  The only
thing you should do is make sure all of the openings are
plugged or taped.  Pull the converter and cover the seal
surface with duct tape to keep the load off the pump seal.
Pull th input shaft and tape the splines good to keep
moisture from them.  You may want to tape the vent on top of
the housing.  Moisture is a bigger problem than position.
It is possible you will have some internal sticking if it
sits too long no matter what you do.

Lugnut

just ensure tht when you re-install the trans., that you let the fluid
work it's way back up and into the seals, bands, check-balls, etc.  In
essense, go slow for a while with the the trans, give the fluid a good
chance to circulate under minimal load.

One local transmission shop told me that they never prebuild transmissions
from cores and store them. They said that seals, etc, can deteriorate under
storage conditions.   Not my words, but theirs.

So it may be not just the position you store them in, but also the
moistening of the seals with ATF, that can be a problem.