The easy way to find out is, look at the regulator or generator Data plate, or cover, It should be stamped 6 Volt..
If not there , pull a headlamp, If it's ever been changed, it should have a number and printing on it like "Sealed Beam, 6 volt" If that does not help pull any other bulb and get the number..translate that number to a replacement bulb, and that will tell you the voltage.
As for positive ground or Negative ground, look at the coil..ignition switch wire is - is positive ground , If it goes to the + Coil it is negative ..
A 1955 should have been a 6 volt Positive ground..BEWARE: It could have been Converted..aftermarket..just a heads up..If it has a stock radio, disconnect it..anything else should operate without permanent damage if wrong..
ALSO beware: Split buss systems (EXACTLY WHY I DO NOT ADVOCATE USING THEM) Can have some tragic Consequences , For a new owner that doesn't know it's been done...if you hook something 12 volt up to a 6 volt ( dropped) buss...Look for a dropping converter/ resistor FIRST!
some 6 volt cars ( and tractors ) were changed to 8 volts,cause other than the 8 volt battery, all you have to do is bump up the regulator. It gives you brighter lights,a little better starting. But it usually means something else is getting wore out.As for the ground, you have to look up your exact vehicle, as most 6 volts were positive ground, BUT there are a bunch that were not. Converting to 12 volts is something to consider; bulbs, battery, Radio, heater motor, and voltage divider (or limiter ) for the gauges , ignition resister, same starter. I did this on my ' 55 Merc about 9 years ago, still running great ! Also , an alternator will charge ALOT faster than the generator !