Well as long as the room is dark most of the time and not in a high traffic area, it might be ok, but honestly a 40 gallon going to be very hard to control temperatures and humidity, so if you can get $50 or more out the tank, get a 20 gallon long or at least something smaller and spend the rest of the money on a Ranco Thermostat ($80 shipped to your door). That way you will never have to upgrade thermostats in the future and have a quality one. I currently have 2 and love them.

What are you using to measure temperatures and humidity? Keep in mind that not all hygrometers work properly, so sometimes a second unit is necessary.