Most modern frames will have a wider stance than the 1950s stovebolts. Tires hanging out of the fender openings will be a dead giveaway that something's not kosher about the chassis. Some people choose to fit big bolt front sheet metal to frame swap jobs, as those fenders and other parts are slightly wider than the pickups and 1 ton front sheet metal. A flatbed will also be a lot more forgiving if you go that route, particularly if you choose to use a more modern rear axle with a wider footprint. The cabs are all the same, more or less, even up through the 1 1/2 and 2 ton models.
Jerry
