be careful what you are doing with regards to running the engine and cutting the power with the engine running. that is basically how they used to get 110 volts from an alternator so they could run a drill or some other tool, back in the day. you could burn up some electronic stuff especially if the dizzy still gets juice to keep running. anyway, think about it and research your particular system. check with a starter/alternator rebuilder, they have switches like you need quite often and can possibly give some insight in to how to wire it. it's a good idea to switch the ground rather than the power just because that is the ground cable so any sheet metal or other steel parts nearby would usually also be a ground. if there is a problem with a cable or a switch and a cable burns off or some other malfunction occurs any loose cables that touch surrounding parts will not cause a spark or short to ground. also, generally, there is less corrosion build up on the negative terminals due to how electrons flow through the system and the negative terminal will usually have less wires qnd connections going to the terminal. do a bit of research on how much total draw you would have at max intermittently and then the usual normal draw with lights, heater, stereo etc, usually that would be starter draw as a max, and ensure the disconnect switch is rated for that much amperage at minimum. a stop at a farm or heavy equipment parts supplier may set you up. possibly something like this.
https://www.amazon.com/Fastronix-Cur...M2G5SM86AMFSCW