Heater hoses reversed won't cause an overheating problem. All the heater core is is a small radiator, once the cooling system is full of water (and no air pockets), it doesn't really matter to the engine temperature which way the water flows through the heater core. As T-Top mentioned, a good shroud is critical too cooling and also where the fan is placed in relationship to the shroud, ideally the middle of the fan blades are even with the opening of the shroud. Removing the T-Stat isn't a good idea, if the flow of coolant is unrestricted through the engine it can flow too fast to do any cooling, put in a quality T-Stat!