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I think you got your numbers wrong. The engine isn't tuned to run rich, at least not more than any other car, you just underestimated the throttle position that gets it into rich AFRs. At lower rpm the throttle position does not need to be very high to load the engine up. Try to get an AFR signal from the OBDII port to see how much throttle you can use before it starts running rich.
Also 3500rpm is way too high. Small 4 cylinder engines often have lowest BSFC point in the 2500-3000 range, the bigger bores on your engine mean the charge takes longer to burn and so for efficiency you want to run it slower, my guess is 2500rpm shifts would do the trick.
All that said I am not sure you will be able to do much better, everyone these days thinks big V8s get okay gas mileage since they are good freeway cruisers but they guzzle down the fuel almost just as fast in the city while you are going a lot slower.
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