Take the thermostat out and test it. I've had plenty of bad ones right out of the box. What kind of a bore is on that 455 and are you running high pro parts? I believe the 74 and newer blocks (correct me if I'm wrong) were not as heavy duty as earlier years. So if your running a sixty over bore with high pro pistons, cam, high compression, whatever, on a 75 block, there may be problems in cooling no matter what rad, pump your running. Also throw in timing too.