There's nothing wrong with using 3/4" tubing on your mag; in fact I believe they're "rated" using 1" tubing. There's no way a mag 7 will pump close to 700 gph at 3 ft head with 3/4" tubing and some elbows. If you use a sqwd, you'll lose lots of flow, which might work out perfect for your overflow; it's worth a try. Regarding the valve, you can get 1" thread by 3/4" hose barb fittings and use a 1" gate valve; it's actually probably a little better than a smaller one anyway. But, the ball valve will work fine and they're cheaper. The only thing about them is that often the valves are really hard to turn on and off; if you're connecting to flexible tube, you'll have to hold the valve steady. Best would be to attach the valve right to the pump; then put a barb on the other end of the valve. You can get a 1/2" X 3/4" (or 1") threaded bushing, put that on the pump, then the valve on it, then your barb. A slightly oversized valve will be easier to adjust, ball or gate.