Are you using a Delco internal regulator alternator ? If so, check if the #1 excite wire is not shorted to the #2 sense wire. If the #1 wire is shorted to the #2 wire, #2 is hot all the time and will backfeed through the idiot light to the ignition coil if both the idiot light and coil are connected to the same ign switch terminal. This will make the light burn when the tractor is stopped.
If not already installed, buy a delco alternator harness repair plug at the auto parts store. This plug makes for a more professional liiking job and is safe from shorts.
If the gen light goes out when you unplug the #1-#2 plug at the alternator, that would point to an internal short in the alternator or regulator.