I had the same exact tank and I was using a MJ 1200. The reason I was using that pump was because I have a custom made stand which is taller. What I did was remove one of the return pumps and dropped it into that section with a tube. Then I cut the tube for the return pump and reattached it back. So that the feed pump to the chiller would sit just under the return pump to the tank. That would mean you have a total of 3 pump in the back chamber.
Take out one of the return pumps and put in a Cobalt MJ1200 that is connected to the chiller. The return line from the chiller is then connected to the return fitting on the back wall. Hope I explained so that it's clear.
You could always run an external pump if you don't have room in your compartment, then you only need to find room for the intake/outflow tubes. I use a Quiet One 1200 (296 gph) externally to run my 1/15th HP JBJ Artica chiller.
Pete, do as I stated above.....put the Cobalt MJ1200 in place of one of the current return pumps and have the return line from the chiller go to the return fitting on the back wall. I'm not sure what @serkel stated above that you'd loose a lot of pressure that way.......It's basically running the chiller with an MJ1200 with the return line attached to the fan-fitting on the back wall of the tank. I can't see how you'd loose any "pressure." The water passing into the back portion of the tank is still being returned to the tank, albeit first passing through the chiller. When I was running the HQI light, this is how I had my JBJ28 set up. Worked perfectly.