The head gaskets don't normaly blow ont he V8's. If you see coolant leaking down under the resavoir, it is most likely the crossover pipe that runs between the heads. The radiator and the water pump are the weak links in the system, either can make small leaks that wont be noticable without a pressure tester. The heater core is another place that coolant can "disapear" since it will leak through the drain.
Find a shop that actualy has the proper pressure tester adapter, its a rather expensive tool, so most shops don't have them for your car.