Personally I would over drill and put in at least 1 1/2 inch bulk heads. I have been there, one inch bulk heads can clog easily! Try getting a turbo snail shell out of one. What can happen will happen. I beleive a 1 inch can handle about 600 GPH overflow. I just don't like them. You could also use two 1 1/2 inch as your main overflows and keep the 1 inch as an emergency back up overflow. Build a large enough overflow box and you could hide them all. Little giants are terrible pumps loud, inefficient, and big. I have gotten a few for customers in the past and they were not too happy with the performance. All the customers that bought Iwaki's have been very happy with them. Get the Iwaki MD70RLT they are rated for pressure and will pump 1500 GPH. I use Mag drives and am very happy with them a mag 12 would do a great job for you also. You will need about a 30 gallon sump or bigger to handle the flow through it also.
Just my thoughts on flow: You can never have too much.