A three row radiator is not ideal for a 400, especially if it's an automatic or if the radiator has not been tested and/or recored recently.

Discoloration and blistering of paint on the center runners of the intake is normal. There are heat crossover passages between the heads (adjacent to exhaust runners) and the intake to aid quick choke opening. These can be blocked off with stainless shims or block plates.

Have you verified your temperatures with another calibrated thermometer? Remove the radiator cap while the engine is cool, place a thermometer in the radiator, heat the engine to operating temperature and compare to the gauge.

Does your engine show any signs of overheating such as pinging, gushing coolant, steam or other external symptoms?

Have you examined and tested your thermostat recently?