I remember such lower line models being used around airport areas for passenger transport for example, most often Checker cars though. Look if you can find any hints on the car for the company that did the conversion; one that comes to mind is Hess and Eisenhardt for example, they mostly did Lincolns though.
Never saw any slabs besides Imperials before; I remember a couple of late 40s harmonica grille Chryslers, Fats Domino or Chubby Checker used one of them for touring.