98 4.3 overheating issues
 

About a month ago my radiator blew a crack in the plastic part. Heads got hot and I replaced them. Now the truck runs. A test drive shows a high amount of pressure in the cooling system. Even after cooling the truck for three hours the pressure poured out the radiator cap. So my original problem is still there despite new heads and manifold. Inspection of of the block showed no visible cracks. Any ideas. Help. Thank you.