You are looking to heat the tank 30ish degrees and keep it heated. If you are bound and determined to put the tank in such a cold area I would seriously look at glass tops and wrapping the back, bottom and possibly sides with some insulation (styrofoam sheets perhaps) to conserve as much heat as you can.
Myself, I'd just put the tank somewhere else but I don't know your situation.
I agree. I didn't see this part of the post somehow. That's going to stress the heater(s) pretty bad - you're going to want to consider this and try avoiding it if at all possible. I'm a big fan of glass tops in the colder months.