Just an off thought, have you replaced the coolant with a 50/50 mixture of water and ethelyne glycol (anti-freeze)? If it's straight water, 100% coolant, or old coolant, those will reduce your cooling capacity.
May want to take a good look at your fan clutch too. It needs to be between free enough to idle at highway speeds so as not to create an air dam and tight enough to pull air at lower speeds and when idling. When you say "heavy duty fan clutch" that concerns me a little bit.
Is there any indication the PO had pulled the heads for a vlave job? It may sound silly but I've seen reversed head gaskets block water jackets.
If you haven't already, a really really good cooling system flush might be a good idea. It can be amazing what some people can put in their radiators sometimes.
Finally, check the pressure on the radiator cap and be sure it is correct for your 360. Too high a pressure cap will not permit the heated coolant to expand into the overflow and coolant, the higher the pressure, becomes less and lkess effective at dissipating heat.
Paved roads, another example of government waste!