When I did my Plymouth pick I ended up using front runners and they fit well to the fenders. The width is max for these trucks but the higher the cab sits the more room you have to play with.

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56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d