I'm liking the way you think. The radiator adds some positive charge to it. Im very curious how it will work. I know you told us but what are you running for lights in there? Wattage, manufacturer, etc. My fan thing will still work in a closed room. The only difference is the higher the ambient the less effective it works... but it still works. Sounds like you over want to move it a little bit. 3 to 5 degrees shouldn't be hard at all. I'd be willing to bet a 4"fan would work just fine. My system is 50 gallons, it will lower the temp by 10 over night, so it's fairly quick also My fan is an Ipower 6" and cost 19 bucks, so it's fairly cheap.
It's working s little too well right now. It's chilly in so cal today. My ambient is 69 and change. Water is, at 59... TOO chilly, but one issue I have is overheating the room. If I mess with the exhaust fan cycle time it may get a little warm so I leave the "chiller" on for safety sake. If it got much lower I would have to do something.
I would say insulate your res. but that is a double edge sword. Insulation keeps the heat in too.
SO! Cars huh? What do you work on? I have an old car, but I can't so I enjoy working on it... haha