At 3500 rpm cruise you could use a smaller pulley without any trouble. My guess is that the factory designed the water pump to be really effective in the 2000 to 2500 rpm range so you are probably overdriving your pump a little right now. A smaller crank pulley might improve cooling on your setup.
I was thinking pretty much the same standard RPM range; 30% slower than 3500 is 2450...
