OVER HEATING WITH A 318 ?

What are you using for an overflow bottle? What is its capacity? Did you fill the radiator completely full of coolant or leave it a little low?

On old systems that didn't use an overflow from the factory, the coolant shouldn't be filled all the way to the top of the radiator. On a stock radiator, there is a small "shelf"(for lack of a better word) directly under the cap. This is where the coolant level should be kept. Even with an overflow bottle, leave the coolant slightly low. If it's filled to the top and you have a small overflow bottle, it's possible to puke out enough coolant to overflow the bottle. Since your temp gauge was reading normal, I wouldn't be concerned just yet.

Drain the coolant out of the overflow and check the radiator level when the engine is cold. Leave it a little low and my guess is you'll be good to go.