Ford used 1/8" dia wire on the cars.
5/32" on the trucks.
I'd use the 5/32" dia wire for your truck fenders.
I have rolls of soft galvy 5/32" dia wire I use in the workshops here for teaching wired edges, because it is easier to roll around larger dia wire than smaller.
I just buy RG45 welding wire in 36" lengths, gas weld the butts and hammer them until cold, making good bendable lengths for any job I have had to do.
Cheap, good quality and available, and the flash of copper helps with corrosion-prevention.
... or it makes me feel better.
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(JagXKE modifications)
Back when I was a pup I was proud of my "jenny" ability on wired edges, until I spoke with one of Breedlove's crew, and I learned the reasons for doing wire edges - by hand - on the car/truck/large section.
I'm happy with the machines doing it but I teach the hand skills.