It is natural to think more water flow will solve over-heating. But actually, a slower flow will let the water stay in the radiator longer, releasing more heat. I had a 1967 Firebird 400 HO, with manual steering. The only fan belt was for the alternator and water pump. It was the biggest water pump pulley I ever saw. But it came with a huge 4 row brass radiator. It never got hot. The condition of the radiator, engine water passages, water pump, and air flow are the real culprits for over-heating.

Do you have the 400 specific air blocker plates at the top of the opening? They help. Do you have a good radiator shroud? Is the fan located half in/ half out? Do you have a working multi-blade clutch fan? I used to steal them off big Pontiacs for the better cooling. I also like to clean the engine block with Cascade dishwashing machine detergent. It gets the crud and rust out. As good as brass radiators are, the new aluminum ones are as good if not better. You just have to use distilled water so the aluminum and iron don't react.

Remember that a lot of small problems can add up to over-heating.