340 with edelbrock heads overheating

bingo. So interference and pre load is the culprit. Glad you got it nailed down.
harland sharp makes a rocker specifically for these heads will be using those first. I think the valves are hanging open a little bit because after checking and checking and finding no vacumne leak its got to be this. I think the added friction is causing it. there is one valve that no matter hoe much i back off the adjuster screw the rocker never gets loose.