503 conversion kit for Ford & GM 203 N.P. (Including 3/4 Ton Hub PN: 423) For owners of full time 4WD vehicles, here is a selection of products that allow for a variety of conversions. Count on these kits to reduce wear and tear on your drivetrain as well as saving at the gas pump. In some cases, these are the only kits available for your vehicle. In all cases, each kit has been carefully designed to provide years of trouble free service.