I've had bad luck with fan spacers using long bolts to the water pump flange, they just kept working loose. That was on a gasser turning 8,000 rpm, but I still don't like the idea of it.
I was thinking of fabricating something like this
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if I can find a piece of 2 5/8" ID pipe with an OD of 2 7/8". The flange to the water pump could be 1/8", while the fan surface should be around 3/8" to allow for more threads. I suppose it could be turned and milled from a piece of aluminum, but there would then be issues with threads for the fan bolts, and the spacer tube itself would be fairly thin for aluminum.
I do like this idea better than using the "big nut" adapter, as you alleviate the threat of your fan spinning off at shut down.