Changing the regulator does not necessitate polerizing the genny.  If you have not replaced the genny just install the new regulator.

But did you test the generator???

Run the engine at about 800 to 1200 rpm.  You can do this by screwing in the idle screw to raise the idle speed.  Then with an assistant watching the amp gauge, temporarily jump the field terminal (the smaller diameter of the 2 genny terminals) to a ground like the engine block with a length of wire.  the genny is good if the amp gauge pegs to full charge.  If the genny pegs then the fault is likely in the Voltage regulator.

But a new battery will not require a lot of charge.  the vr's job is to allow the genny to replace current used like in starting.  but if the battery is new and at full charge, it may not require a lot of recharging so the Vr will not engage genny for very long or at a high rate.

My car runs down the road with the gauge just about a needle's width to positive after about 2 minutes of running.

If you feel you must polorize the genny, it done like this

I am new to Plymouths and their Positive Ground 6 Volt system, so I need some help. I need to "Polarize" my new Generator Regulator, but what I've found and read is a bit confusing.  The car is a stock 1954 Plymouth Belvedere with it's 230.2 ci engine.  I put in a new Battery, but it doesn't seem to be Charging, so I'm going to replace the Regulator to see if that helps but I don't understand what to do about the Polarizing.  I do have to original Service Manual and a 1954 Motor's Repair Manual, but I don't understand the process clearly. 
What do I have to do to properly Polarize my Generator Regulator.  Thanks.