Any auto trans that essentially sits still for that long is bound to have dried up seals and O rings throughout the trans.
That leak looks like the front pump seal has gone hard and is leaking, and or, the pump O ring seal has dried out and shrunk, allowing the oil to leak out.
You mentioned the trans was empty of oil when you picked it up. Sounds like it was parked due to the leaking oil situation and you have inherited the situation.
You seem to be mechanically inclined.
Have you ever resealed an auto trans?
TH350 transmissions are not difficult to rebuild, and there are several great threads on this forum to guide you along the way.
You could always send the trans to a local rebuilder and have them reco the trans.
BTW, other leak prone areas are the dipstick tube, the rear seal on the output shaft and the governor cover (the metal cover that looks like an inverted bowl on the side of the trans). All of which would be renewed on a rebuild (or at least on one of my rebuilds!)
Have you driven the truck? Does the trans shift and drive o?
