I would have to see schematics and maybe inspect the wiring of the car to identify the changes needed to accomplish this change. On the surface it would seem like a simple change, just connect the old postive side cable to the battery's negative terminal and vice cersa, however, because of the way the circuits and components are wired and operate, there could be serious (unsafe) issues to deal with.
IMHO - a car with 60 year old wiring
would be a candidate for a fresh wiring harness, with that in mind I would consult with painless wiring (or others) to identify new negative ground wiring harnesses and components to build a new electrical system. This avoids both, old wiring and conversion hazards.
(BTW - I would change the ammeter over to a voltmeter).