The engine is as "grounded" as it's ever going to be with the negative battery cable at the starter housing. That draft tube bolt would be a good place to attach small diameter ground wires from the cab and the frame. That bypasses all the rubber mounts- - - - -engine, bell housing, cab, etc. so the lights, instruments, radio, and heater blower have a good ground. "It ain't rocket surgery!" All of the electrical circuits need a clean, low resistance path back to the (-) side of the battery. It's also a good idea to attach a ground wire between the bed and the frame so the tail lights and turn signals aren't searching for a ground and causing problems.
Jerry
