Uhm.. maybe you should go with a 600GPH overflow.. which would be a one inch bulkhead.. but for a tank that big maybe you would benefit better from a 1.5" bulkhead, or maybe two 1" bulkheads.
As for the return pump, you need to calculate the height from where the return pump will sit to where the return line goes over the lip of the tank, if you're not drilling the tank to begin with. Then, once you know what that height is, look for a return pump that is stronger. As an example, if it's 4ft from the return pump to the lip of the tank, then you would most likely want a return pump that can push water up 6ft, in order to give you extra flow in the tank. Because if you just got a pump rated for 4ft of head height.. you will have very little pressure coming from the return line. Just enough pressure for the water to just make it over the edge of the tank.