Check the cooler and lines. When I built the 440 for my coronet, I had the car parked in the lot at work with the radiator out and the lines propped up to keep them from leaking. After installing the engine, all seemed fine until the first time I took it up to highway speed. It then puked at least a quart out of the vent. After verifying the fluid level (I had installed a new tube and locking stick), I finally decided to flush the lines and cooler. Found the lines plugged with small insects called cutter bees that had climbed into the lines while the motor was out. Blew out the bugs and all was well.