All headlights out with other lights working points to headlight relay or combination switch failure. Check and swap headlight relay first - if no change, the combination switch is likely failed. Common issue on late 90s Ford chassis.
A voltage drop from 12.5V to 10.3V under load indicates excessive resistance in your headlight circuit, likely from corroded connections or undersized wiring. This significant drop explains your dim headlights and requires immediate attention for safety.
Converting pull switch to toggles is straightforward. Label all wires, match switch ratings to circuit loads, and wire power to both switches with separate outputs for markers and headlights.
Your 1995 F53 460 7.5L is stalling due to a failing ignition control module (ICM). Since you've confirmed the spare ICM fixes the problem, replace the faulty module and carry a spare for peace of mind while traveling.
LED clearance lights turning off when headlights activate is a ground issue. LEDs draw little current so poor grounds cause backfeed. Fix by improving ground connections.
To connect LED halos to your F53 DRL system, locate the DRL output wire (typically light green) at the headlight connector and splice it to your halo positive wire, maintaining the existing relay control upstream.