The trans will store pretty much forever as long as all the holes are plugged- - - -fill tube, case vent, and a slip yoke plugged into the back of the case to seal the output shaft hole.

Here's a trick for long term engine storage- - - - -drain as much oil as possible, seal off the intake and exhaust with wood or metal (not duct tape) and turn the engine upside down. The residual oil will creep past the piston rings and keep the combustion chambers wet, preventing cylinder rusting and other nasty stuff caused by a few valves being open to the atmosphere.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!