contecar4u,
If you have any concerns, just leave the ignition off, and disconnect one of the terminals while you are charging the battery.
As for the car being positive ground and charging it, there should be no concern. I have several antique cars, all of which are 12 volt, except the TBird is 6 volt.
My 1954 MG TF is 12 volt but has a positive ground. I simply charge its battery during storage the same way as all of the others (just hook up the charger leads as you would normally do, + to + and - to -). I do however, have a battery disconnect which cuts the battery out of the electrical system when the cars are not in use. That way, no short circuits caused by mice or other manners can start a fire. Just be sure to pop the cell caps off during charging, to allow vapours to escape.
I've had absolutely no problems charging any of the batteries once a month when they're not in use, for the better part of 15 years or more.