Personally, I'd use an UTH on one end of the tank. I typically use the 11"x11" size on 40 breeders and up. Not sure on the wattage for those, but I typically use the 8"x8" size on smaller cages. I have the extra security of using thermostats on those as well to control the temps better.
I don't quite trust using heat lamps. They just don't seem that safe to me, couldn't tell ya why, just a feeling I have about them. Not that others don't use them quite successfully. One problem with corns though is that you'd want to use a nocturnal bulb if you're using it as the heat source as you'll want a day/night cycle of some kind in order to not stress Ruby out!
D80