More thoughts. On the hot one(the six pack) I'd (1)wire the heat riser open & R&R the intake & block the passage & try it out (2) R&R the clutch fan w a fixed blade one & try it out (3) pull the stat & try it out (4) rejet the center carb 4 sizes up & try it out (5) bump the initial up 5-7.5 degrees & try it out (6) jack up the front end high, take out the stat & rev the eng several times to possibly push an air bubble up & out just like burping a baby(hey it's after midnite here).(7) have a radiator shop check that there are no combustion gasses in the coolant