I did almost the exact same thing on my 37 gallon All Glass tank (30" long x 12" wide x 22" high, with tempered bottom which cannot be drilled). I drilled for two 1" bulkheads near the center of the back wall of the tank about 3" below the tank top, then drilled and installed an overflow box from an overflow kit (the kind of kit with the "U" tube) inside the tank on the back wall over the holes. These drain to the sump at about 400 gallons per hour. Then, I drilled for two 3/4" bulkheads, one at each end of the back wall of the tank, for the returns. I used modular piping for the returns, so that each 3/4" bulkhead connects to modular piping which runs the full 12" across both sides of the tank. I put a 1/2" modular piping outlet nozzle on each of the four conrers of the tank. I paid a glass shop about $50 to drill the four holes (but it was at my own risk). The overflow box was $50. The four bulkheads were about $40. Works great. Never had any problems in almost two years. I prefer using the overflow boxes this way rather than the usual overflow boxes supplied by the tank manufacturers (which run from top to bottom in the tank) because (a) they take much less room in the tank and (b) I don't like the idea of having the overflow take in water at the level of the sand bed, which most do now. I like the modular piping for the returns because (a) their direction(s) can be changed easily and (b) no powerheads are necessary. I am upgrading to a 55 gallon this summer and will probably be using Sea Swirls to get more erratic movement. But if you don't want to use Sea Swirls, I think doing it the way you've/I've described is a great way to go. Good luck.
