That one, I don't know. However, he wasn't (isn't) the only person to run full-legal-limit from a vehicle. I've even done that! But, back in the days of AM, it was somewhat tougher than it is today. In the photo gallery of my web site is an album about old radio, and three photos on a 50s Mercury wagon replete with trailer mounted generator.
To date, the highest powered mobile I know of, has a CW output of just under 6 kW! It's in a modern crossover and uses an Aura Systems unit to generate the 3 phase, at 8,500 watts, to power an Emtron DX4.