Let me be the devil's advocate here. Before you start swapping or adding fans, I would suggest that you make sure that your radiator and other cooling system components are in tip-top shape. Furthermore, don't let that silly temp gauge scare you, they frequently indicate higher temperatures than are really there. Take some actual temperature readings on the coolant and/or block. (Lots of stuff on that in the BB archive) BTW, I've swapped two-bladed, for four-bladed, for five-bladed, and for six-bladed fans without any noticeable difference in temp gauge readings.
I drove Amazons in the tropics for decades; jungle, desert, mountains, you name it; and never had one boil on me, excxept when my radiator hose gave up the ghost, and then the only thing damaged was the temp gauge - the engine went on for another 100K miles or so before before it started to use a little oil. True, those were B16s, but I don't believe B18s or B20s are less happy at high temps. I would only mess with fans if, after I had had my radiator thoroughly cleaned, I could get the thing to actually boil, and than I would still think the problem might lie elsewhere - like tuning.
Bob S.