I would agree with most of the above, but not all.
At the power levels you are probably targeting and a radiator that size, ram airflow is most likely sufficient at speed. With trucks pushing more power, thinner radiators, or smaller frontal areas the fan becomes significant at higher speeds. That said, your radiator appears to have the ports for an automatic transmission. If you are running an auto, you will be dumping more heat into it, especially carrying heavier loads, and that will have an impact.
I have had better success with electric fans that mechanical ones. The clutches on mechanical fans fail in non-obvious ways and it can be a real pain to track down what the problem is. An electric fan has more obvious failure modes and (in my experience) is less prone to failure.
The electric fan also has the advantages that it can ramp up to full speed while you are stopped, giving you full cooling for yoir engine and/or AC. Additionally it consumes no powerand makes no noise when it is turned off.
An electric fan does require more wiring and a sufficiently large alternator to support it, but if you are going to an SBC anyway it is unlikely that you would find an alternator small enough to be a problem.
