Technical AC Tach connection question

I have a question regarding the electrical connection of an older AC tach that I would like to use in a flathead V8 powered car:
As far as I know the tach is most probably originally from a boat. According to a stamp on the side it is 12V and for 4 cylinder engines (so it will not read correct without adapter, I know).
Unfortunately I can not find any description how to to connect the tach to the motor's electric system anywhere. Do I need a tach sending unit like the old SW tachs use or can I connect it directly to the ignition coil? Any help/information is highly appreciated!