cooling my slant 6

The four blade fan should be fine. If you have reservations about it, try a viscous fan and a shroud from a car with A/C.

While tuning up the cooling system, check the lower radiator hose. I had a problem with the 73 /6 overheating at highway speeds (70-75 mph). Turns out that as I increased speed, the water pump's suction would collapse the hose. Part of the problem was that the spring in the hose had broken.

Another thing to check is the radiator cap. If coolant is kept under pressure, the boiling point is higher compared to coolant that is exposed to normal atmospheric pressure. IIRC, a properly functioning radiator cap will release somewhere around 16 psi. Don't use a NASCAR style 25 psi cap, it will probably blow off hoses secured by spring or screw clamps. lol