[bobcowan]

A lot of this is simple math. A mustang radiater has a surface area of about 351 square inches. A 16" fan covers a surface area of about 201 square inches.

If you simply attach the fan directly to the core with nothing but a gasket, it will only pull air through about 67% of the core.

If you mount the fan to a good shroud, it will pull air through 100% of the core. It will also be slightly more efficient because of the ventouri effect.