Despite your efforts in this endeavor I don't think you can accomplish what you are trying to accomplish. If I correctly gather what you are tyring to do here, it sounds like you are essentially trying to make a cyberQ controller out of the ebay aquarium controller.
The first (and biggest) problem is you are dealing with a heat source that varies because it depends on drafting air to stoke the fire and thereby increase or decrease the heat. The CyberQ acccomplishes this through its software that depends on very precise mathematical calculations. The way it works is it detects a temp drop and kicks on a fan at various speeds for various durations to bring the pit up to temp and keep it there. In otherwords, its not just on/off, its on/off, low/high and long/short. The software is really what you are paying the big $$$ for. It's not so simple as taking a temp reading and kicking on a fan.
The second problem I think you will have is this: a 274 gallon tank is a big pit. I don't know how thick the steel is or how tight your doors are, but even a CyberQ will not produce ideal results because it requires a tight pit to accurately regulate the temps. In otherwords, it has to be able to draft air at a constant rate through the baffles. If you have leaks around the openings you can have the baffle closed all the way and the pit will draft air. A cyberQ can overcome this, but it is my understanding that you will burn through alot of wood. CyberQ's also work best on smaller pits. Something your size would require at least two of their larger fans to pull enough air...and again, that means burning alot of wood.
I don't have any real details on your pit design but I'm betting it is an offset "stick burner." The reality is that type of pit requries alot of baby-sitting. I should know, that's what I use. If you don't like all day sit by the pit Qing, invest in something like a Weber Smoking Mountain, a Traeger pellet smoker, or one of those gravity fed cabinet smokers (backwoods for instance) if you want a "set it and forget it" type of setup. As for me, I enjoy sitting by the firebox with a brew in hand baby-sittin my stick burner while I smoke Brisket and Spares!