My 1965 3000 is neg ground and grounds from the - terminal of the battery through a size 0 wire to one of the bolts that help to hold the steering housing onto the tranny housing, right side of the tractor. + terminal of the battery is available on the left side and with about a 12" size 0 wire, connects to the outboard terminal of the solenoid. Inboard terminal goes to the starter stud.
Don't know why they would all of a sudden go back to ancient history and do a positive grounding setup.
As far as the inst. panel is concerned, you'd have to reverse the power input and ground wires for the meters to deflect to the right in the normal fashion if someone cobbled up the wiring. On the tach and idiot lights it matters not.