It may be hard to tell with a new engine, but an engine running above ideal temps starts to sound ragged. If it's really hot, you will here "ting" sounds from the exhaust manifolds, brackets, and other pieces as they cool. I suspect that your sending unit and gauge may not be well calibrated, but just to be safe...

1. What type of sending unit are you using...one piece gauge with thermocouple, or electric sending unit? If electric, is the sending unit designed to match the gauge? If not, your gauge reading will not be accurate.

2. Ditto on the thermostat. It's easily overlooked. I usually drill an 1/8" hole or three in the thermostat plate as well to aid in the removal of trapped air.

3. Does the water pump have cast or stamped impellers? Did you check the clearance between the vanes and the backing plate?

4. Depending on the age of your radiator, it may be partially clogged. The water passages build up mineral scale and close off circulation. What may have been enough cooling capacity for a 350 may not be enough for a brand new 455. You can remove the coolant and have a radiator shop inspect it. A few hundred dollars for a new radiator will pay itself back many times over.

The water level in the radiator sounds a little low. When fully cool, add coolant to the fill line, about 1 1/2" to 2" below the neck. Set the cap in place and squeeze the upper radiator hose. You should hear liquid sloshing from the tank to the water pump and back. If not, add a little more, but not full. You should be able to see the liquid moving in the radiator when you do this. Then close the cap securely. A very small amount of coolant, one cup or less, may dribble out after the car reaches operating temperature. Leave the radiator sealed once it does.