The Factory wiring on the 300-6 the negative battery cable goes from the battery Negative terminal, and goes directly to the starter mounting bolt where it bolts to the engine. If the starter mounting surface to the engine is not really clean or good, this can cause a poor ground to the engine.

Also midpoint on this cable there is a tab that attaches the cable to the frame. This is how the frame is grounded from the factory.

Make sure these are clean and tight, and present, as replacement cables do not have the tab on the cables to ground the frame, so you will have to improvise something.