| Is it 1100 at the head, or at the pump? Easiest way to test this (that I've found) is to get a one gallon jug, a 5 gallon bucket, and a stopwatch. Go in your driveway, fill the 5 gallon bucket with water and put the pump inside. Measure how high the head will be in your tank setup and keep it the same height while you're testing in the driveway. Have someone else hold the stopwatch and plug the pump in while you hold the jug and end of the hose at the appropriate height. Start the watch when the water starts flowing into the jug and stop the watch when the jug is full. I did mine 5 times to get a good average on the stopwatch. Then with some simple math you can figure out the gph flow rate and decide how big of an overflow box you need then.
My pump rated at 640gph flows about 400 with a 4ft head and 1" tube. I have an 800gph overflow box with a hofer gurgle buster that keeps it pretty quiet, despite several 90degree bends in my piping | 
