"shop manual says that once the truck over heats that the sender/gauge need to all be replaced." pardon?? where exactly did you read that? vehicles overheat from time to time, especially these older rigs, replacing the gauge every time that occurs is definitely not required
the rad doesn't "move coolant" the water pump does that, the rad passes it top to bottom and cools the water in the process - you should find with a laser thermometer that the bottom rad hose will show 5-15* cooler than the top - if the flow in the rad is slow, it's sludged up and needs rodding out, a process unknown to many younger folks at newer shops, maybe you should check with another rad shop that understands the old systems .... and if the system "is not moving all that much water" you might want to check the water pump
if the water in the top rad hose is about 185, then you have a 180* thermostat, and the gauge is showing about 185, regardless of what the numbers on it say .... these old gauges are not scientific grade fine instruments, and while it may be that yours is so inaccurate that you should replace it, replacing the whole cooling system is a solution to a problem you don't have, particularly as you say money is tight .... a post in the parts wanted would likely find you a good used gauge with the sender bulb still attached
Bill
