The LSD does not effect MPG. Toyota put the 4.3 in the N/A cars because the 3.9, 3.7, etc does not get you the acceleration from a stop.
You won't like your car with less than 4.1.
You also won't gain any MPG, because you will be giving the engine more fuel to get you going from a stop.
It isn't posible, but the best way to get more MPG would be to have a 6sp tranny. 6th being a dbl overdrive.
Corvettes do this, and they get 30MPG doing 80mph (personal experiences)