Typically, rule of thumb for a return pump is to turnover your display tank 3x - 5x per hour. So, you should be looking for a pump (after head pressure) that will run at around 350gph, not 1200gph. Your don't want to rush that much water through your sump, so keep to less than 350gph. If you used a pump rated 1200gph, the overflows would be unbearably noisy as well and bubbles galore returning to the tank. All the sump water may fill up into your tank, that wouldn't be funny.
Even the Eheim 1260 and OceanRunner 2500 might be a little too much for your setup. You may need to add a ball valve on the ouput and reduce the flow a bit.