It's not the absolute temps that will cause problems, but rather, the differential between the metal and the ambient air temps. If the metal is kept even slightly (1 degreee is theoretically enough) warmer than the ambient temp, no moisture will condense from the air onto the metal surfaces. If the metal surface is cooler than the ambient air, you will get condensation and rust. A GoldenRod is adequate to maintain that differential in a normally sized safe, and is more reliable than a light bulb, because light bulbs burn out when you least expect it. Those who cautioned about bringing cold guns into a warm house are correct. You might also consider insulating the outside of your safe with a fiberglass blanket (a big water heater blanket can be used). If you don't open the safe often, this will minimize the chances of temperature swings inside the safe. You could also run a strip of thin weatherstripping (EPDM) around the inside lip of the door to minimize air infiltration and heat loss from the inside of the safe.
Seems to me that you will be most at risk when you heat the garage with the wood stove. The guns in the safe will warm up much more slowly than the air in the garage, and that will create condensation risk until the metal temp equals or exceeds the air temp. You could put a tub of dessicant in the bottom of the safe, but most dessicants work pretty slowly.
