With the Delco internal regulator alternator, following the wiring diagram in the link below should help you wire it correctly.
If it kills the battery when you shut it down, sounds like your #1 alt wire is connected to constant power, and is not turning off with the ignition switch as it must.
Also, it showing a 20 amp discharge while running, MAY be a discharge, or really a charge if the tractor was switched from pos ground to neg ground without switching the wires on the ammeter from side to side (post to post).
If the ammeter is connected wrong for the battery polarity, it will indicate a charge as a discharge.
One way to determine that is to switch on a load with the engine stopped(lights, ignition etc.) If the ammeter moves to discharge when the load is switched on, then the ammeter is wired correctly. If it moves to the charge side, then it is reading backwards and needs the ammeter wires switched from post to post to make it read correctly.