Ignition problem: starts and runs in neutral but dies off in park

Either you have a wiring problem such as:

Loose/ bad connections in the bulkead connector, a failed splice on the run wire in the bay, the ignition switch connector or the switch, or maybe the ammeter connections "in that area"

OR you have somehow tied the START WIRE TO THE IGNITION circuit. In other words using the yellow start wire to provide starting voltage, or using the brown bypass circuit tied to the start relay

WHAT MIGHT be happening: In the above scenario, you have the "run" circuit tied to the "start" wire hooked to the START RELAY. When you put the trans in park or neutral, the start relay puts an additional load on the ignition circuit, loading it down through the relay and neutral safety switch, which is "made" in park/ neutral.

Is the ballast still in place?