400 BB build issues

Nowhere near enough radiator. That's the original F body radiator I see.......lose it for something better. A LOT better. RUN A THERMOSTAT. Put a 180* Mr Gasklet high flow thermostat in it. They work GOOD. Where on God's green earth do people come up with the idea that not running one is a good thing? Put a BIG box fan in front of it for break in. The more air flow the better.

IMO, if you have no white smoke, no water in oil, no oil in water and no skips you can RULE OUT a blown head gasket or cracked head. But if you keep running it with a half *** cooling system, that won't be for long.