it's actually possible. ford factory workers did have a tendancy to load their own personal cars up with the the best of everything. i've heard of factory workers who had the engine guys build special balanced and blueprinted engines with the best factory components and even had them make special "modifications" using experimental or over the counter factory speed parts, blueprinted transmissions and differentials, sometimes even parts from completely different model lines and other "unauthorized" options. sometimes even "lunchpail parts" would make their way onto these cars too.
so all in all i would say, yes, it's possible that the car had special frame modifications however i would make the seller prove what mods were made and back it up with pics. i would think that about the only chassis mods that could be done would be to use convertible parts, like floor pan reinforcements, inner rockers, torque boxes, etc.