For your overflow plumbing, you can get away with 3/4", or go with standard 1" that most people use (but on larger tanks). Personally, I wouldn't use a Durso as I find them not totally quiet while introducing bubbles into the system. An easier and better solution is to simply have an open-ended top drain and use a gate valve below the tank to choke down the flow. Once you have it adjusted properly, the water level in the overflow box should maintain steady at a few inches above the drain. This results in a perfectly quiet, bubble-less overflow.