727 trans leaking in vertical position

Well sure. With no converter, there's nothing for the seal to ride against so fluid just runs out the front seal.
As stated, the converter is in it, fully seated. The trans was rebuilt shortly before removal and only had about 900 miles on it, with no problems while it was in the car. Been sitting for about three years. Wouldn't expect the seals to fail in that short of time while it was sitting in normal operating position.