Are you sure you have a Clark model C500-50? That model number indicates it is a basic 500 lb. unit (you've stated it has a 400 lb. lift capacity) and the Wauhesha engine D155 G is an overhead valve engine that Clark private labeled (the Clark name is cast in the rocker arm cover). The flat head engine that Clark used was the F163 in there C series & C300 series in this capacity range.
A serial number can help identified what you have, this is on the truck number plate & stamped in the frame and is in a format like ****-yyyy-zzzz.