340 with edelbrock heads overheating

If all you changed was the heads , Then I would do a leak down test to see if the valves are sealing. Did you check intake fitment? Could it be sucking air from the lifter valley?

Seal off the breather holes in the valve covers and the PCV hole. Also plug the PCV hose. Start it and see if there is a difference with the valve covers sealed. This will tell you if its sucking air internally