Putting Fuel Injection on the motor isn't going to solve your engine problem. A properly operating thermostat will open/close to control the engine temp. I don't care how efficient your cooling system is, if the thermostat shuts completely, you will not have coolant flow through the engine and it will get hot. Either you have an inaccurate gauge (stock or aftermarket?), or a defective thermostat that is not closing when the motor starts cooling off on the highway. Have you tried taking the open thermostat out of hot water and placing it in room temperature water to see if it closes, or just sticks open?