It sounds like you don't want to revisit the overheating but I would like to mention a few things. I'm more familiar with my '68 400HO with AC but I can't think the '68 is much different.
1st, you mention a recore on the radiator being 3 cores. A 400 with AC should have received the most robust radiator from the factory. I get confused by radiator rows verses cores but I thought mine has a 4 core Harrison UP code.
Do you have the seven blade fan with the clutch that gives it a high and low speed above and below 150F? Is that fan about half way into the fan shroud +-?
Do you have all the three baffles and two seals that direct air flow into the radiator?
You don't want to discuss it. that's OK. It's just that my '68 with 400HO and AC has never overheated in 188k miles. (Other than one stuck thermostat.)
