I agree with Ed, you have some sort of cooling system problem that needs addressed. A bigger fan isn't going to fix the problem.
Another thing you might look at, since I see you're running a '55 engine, is the water pump relocation plate. It moves the pump up to the stock location for the original 216, which also centers the fan in the radiator. If you have already done this, or have the early '55 high water pump engine, then disregard.
