It might help if you were a little more specific on which bolts you're referring to. Inner (splash) to outer fender? Fender to cab? Upper brace to fender at radiator support? Fender to brace at the fender lip? I'm no expert, but the largest clutch head screw I've seen is a 1/4"-20. All the larger fasteners holding the fenders on are 5/16"-18 IIRC. A few years ago I came across a early 50s AD sitting in northern Arizona. It appeared to be unmolested since original and the bigger fender bolts were six point with a star washer. I could probably find some photos. There's some links to manuals over at the Stovebolt website. You might find what you're looking for over there Tech Tips for Restoring your old truck
Hex bolts. I have a number of AD trucks and all the bolts on the front sheet metal clip are hex except the ones holding the upper grill air baffle and the hood latch striker plate. Of course they are all over 60 years old and can't talk so the factory assembly manual would be the most reliable source.
