It seems to me the "screw in the overflow pipe" trick will solve your water loss problems. The water expands and flows out of the overflow pipe, then when you slow the car down, the volume of water in the engine drops because the margin necessary to keep the radiator filled had poured out the overflow pipe because of expansion and the volume of water pushed into the radiator by the water pump at higher rpm's. Of course this is the idea of the overflow tanks on modern cars to keep the radiator filled, what is pushed out is drawn back in. There is no way to save it in a container because the system is "open" and not "closed" as in modern cars, and the overflow systems have a 15lb cap on them.