Strangely enough, the correct or proper Ignition Coil polarity has nothing to do whatsoever with whether the car itself is positive or negative ground.
It is true that hooking the coil up "wrong" will result in a less than optimal spark. Some coils marked "+ and -" and some coils are marked "BAT and GND" but either way there's a question - is the coil itself one manufactured when positive ground was de rigeur, or, is it a post 1956 coil that presumably is marked different?
Fortunately there is an easy way to determine if an ignition coil is connected correctly regardless of the markings on the coil, or the vehicle polarity. I suggest using the analog style voltmeter. It only takes a second, and there's less risk of zapping yerself. See:
https://mgaguru.com/mgtech/ignition/ig104.htm
Why then do we worry about coil polarity? Because the spark plugs do care which way the electrons are flowing in the high tension circuit. The spark plug has a thermally insulated center electrode (surrounded by ceramic). With engine running the center electrode runs substantially hotter than the exposed end electrode. Design of the ceramic insulator determines how hot the center electrode will run, leading to the designation of hotter or colder spark plugs. As electrons go, they love to jump away from a hot surface and fly toward a colder surface, so it is easier to drive them from hot to cold rather than from cold to hot. End result is a difference of 15 to 30 percent in voltage required to make spark "initially" jump the gap on the plug depending on which way it is going. So the spark plug prefers to see a voltage potential that is negative on the center electrode and positive on the end electrode for the very first hop of the spark. Oddly enough, this has nothing to do with polarity of the vehicle electrical system, but it is influenced by the common connection inside the ignition coil.