For the "chiller" I'd tell you to put a large cooler filled with water in there. Then run the water tubing throught he water. The specific heat of the cool water will be a much better cooler then an air-to-water setup you have now.
8' x 4' x 2' = 480
12' x 4' x 3' = 1078
If you got the extra 4 feet in width and can go a foot taller I say do the 1000 gal.
The larger the tank the more stable it can be...