If you have DRL,you can simply unplug the module located on the drivers side inner fender,and they will no longer work,and headlamp operation is unaffected. We have seen a few modules die on some of our trucks when connecting and disconnecting our snowplows. You could also run a toggle switch from ground to the parking brake switch which will turn them off when you wish (although the brake light will stay on).
Which brings up another point. If you have DRL (and even if you don't,it won't hurt),when connecting or removing a snowplow with the type of headlamps which switch over automatically,always make sure that the truck is OFF,and the headlamps are OFF. We have seen three damaged by the voltage spike when this happens. All of them fried the DRL module,causing the highbeam indicator to stay lit,and the headlights would stay on even after they,and the truck,was shut off.
If you don't have DRL,but would like to,to get your insurance discount,you can just wire up a relay to turn on the lowbeams (or some people use the park lamps\fog lamps) when the ignition is in the run position. We have done this on our older GM plow trucks to get a discount. We connected the relay to the choke wire (fed via oil psi switch),so they only worked if the truck was running). I'm sure you could rig this up on a cummins as well.