I think I have decided to hook up all the heat ducts and box while I have the dash not completely assembled, and with the drive season over. I have a heater box and dash controls out of a '73 super duty car with heat only. How different is this box from the 77-81 style, and are all of the heat ducts the same? I only have the box, and need to locate all the heat routes, and am wondering if it might be easier to put in the '77-81 heat box instead....Any thoughts?