A big block chevy with, I guess iron heads and 12.1 compression
is asking for trouble on pump gas. Your cam has 240 degrees of
duration @ .050 is also hurting. You have to much cranking compression which is why you're running hot. Are you exprerimenting detonation also? A cam that has late valve closing
(ABDC) would bleed off some cylinder pressure, that can help. Bottom line is, your setup needs racing fuel. Wrong combination for pump gas.