The current regulations were ICC regulations back when your vehicle was built. The proper name for the light on each side is clearance lights. The 3 in the middle are identification lights and may be no more than 6" apart with the center one in the center of the truck.
Had your vehicle been used in interstate commerce at the time of manufacture, it would have been required to meet the same regulations as today. Why not just go with what was legal then and is legal in all states now
