For maximum effect the fan should be about 1/2 way in the shroud opening.
How many blades on the 17" fan and what style????
How much distance between the tips of the blades and the shroud ID????

Another thing to try is to put a towel on the front of the radiator with the engine idling to try and establish how much air is being drawn across the core.
I agree with the others that it the symptoms indicate inadequate air flow through the radiator