There is nothing wrong with the SM420 you have. The engine and transmission will be a simple bolt in. It will stand up to a 350 and it has a PTO opening. PTO's are no longer made for it, but they are around. Almost every GM farm truck in the midwest had a pto for a dump. I think there is a guy on this site who has one advertised for sale. http://www.oldgmctrucks.com/