I flat towed a 60 F100 behind a car trailer on which the 62 F100 parts truck rode, all pulled by a 2500HD. Kept speeds to a reasonable amount and had lights all the way to the back. WI state patrol followed me for a bit but didnt stop me. Doing it this way saved me an extra 12 hour round trip.
Long story short, flat towing your big truck with the half ton should be fine if your speeds are reasonable and you keep in mind your braking distances.