If you run just the T to the headers you will have too much water in the tubes. Some systems also run another exit through the headers at the back, or just overboard like you mentioned. IMO, using just the T will not allow enough water through the engine to maintain cooling. Rig up another "dump" hose or 2 and send it overboard. I put hoses out the back, which I now wish were out the side near the driver so I could monitor the water temp with my hand as it leaves the boat. I also found that even 2 dump hoses were not enough to reduce flow through the headers, so I put reducers in the fitting as it splits to the little tubes. There is a fine line between enough water and too much as the RPMs change and the pump pressure comes up...........MP