I remember reading that GM and Ford had an advantage over Chrysler and Packard after WWII. The reasoning was that GM and Ford build trucks and tanks during the war while Chrysler and Packard build planes, making GM and Ford better equipped to return to normal production once the war was over. Is there any truth to this?
So part of your question is on a shaky premise, that GM and Ford did not also build aircraft.
GM for instance built thousands of F4F Wildcat Fighters(as the modified FM1/2 designation) while Grumman switched to the Hellcat. These Wildcats served in the air wings of the smaller light and escort carriers, notably in the battle off Samar.
North American was also a subsidiary of GM at the time, and their stable included the B-25, the ubiquitous T-6 trainer, and the famed P-51.
While FMC constructed the Willow Run plant which amount for about 8,000 B-24 Liberators during the war, or just under half of the total production run. Along with setting up a factory in England to produce aircraft engines.