There is a limit on how much air that can go thru a radiator. Also in my opinion, having a fan in front of the radiator, actually limits that amount of air that can pass to only what the fan can push.
Like was said, you need to look at the cause of the overheating problem. If you dont have a fan shroud, get one. Have the radiator checked for proper flow. Is the radiator the right one for that set-up? Check your spark plugs to make sure the engine isn't running lean. Is the thermostat the proper one for that engine/set-up? Was the engine bored out too far for that block? Is there too much grille blocking air getting to the radiator? Find the cause of the problem, then you can fix it.
Overheating is a symptom of another problem. Don't treat the symptom, fix the problem.