My 408 ran12.05 with LAX heads and hughes 228/232 hydraulic cam. 750 Holley, RPM intake. I estimated it at 400 hp. I'm thinking you should run mid to high 12s. 900ft altitude here.
Heating up at idle is most likely insufficient air flow through radiator
My temp went down 200 to 180 at idle by adding a decent shroud and a new fan clutch.
The low temp at cruise is a thermostat.
Good luck Bryan