Maybe only 3500 long wheelbase Dodges per year, or was it 3500 for the entire Mopar line?
Many of the body parts were common to other body styles. Fenders. Cowl and windshield. (except convertibles and station wagons). Trunk lid and stampings, and rear window, common with sedans. Many other components were common. Need longer frame, with heavy duty running gear. Add an 18-inch drive shaft between the standard propeller shaft and the transmission. Longer body required, with floor panels, rocker panels, door and frames, and roof panel. No big deal, if they can sell enough.
Long wheelbase sedans were common for taxis, funeral processions, and hauling people to and from hotels, resorts, etc.
So, if Ma Mopar is getting Briggs bodies for Chrysler and Imperial limos, and DeSoto taxis, why not make some long wheelbase Dodges?