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properly sized headers could help as they have smoother curves, but over sized headers will hurt you, study fluid dynamics and you will understand this better, otherwise if you look at the cars that get great gas mileage they all have smaller exhaust systems then their lower mpg brother of the same displacement engine.
An electric fan is going to be lighter, and only run when the coolant coming out of the radiator is still to hot, but converting mechanical to electrical and back to mechanical is not efficient, so it depends on if you have a big engine that is under load alot, or if you never load up your truck, in other words, if the fan is on nearly all the time then it should be mechanical, if it only comes on when you tow a trailer, or go up a mouton, then it should be electrical.
a lighter weight fly wheel gives you less torque, and makes it harder for your engine to run slower, but if your engine speed varies alot, like driving in the city, or racing where you might shift alot and rev the engine higher, then a light weight fly wheel is perfect.
under drive pulleys are designed for racing, where the engine is turning enough faster that you can afford to turn your alternator slower, they are often aluminum so they are lighter, but they are also smaller, v-belts have a minimum size pulley that they can go around, the larger the pulley they go around the looser the belt can be, and the more efficient the whole setup can be, the belt will also last longer as it's not bending as tightly, so instead of going with a smaller engine pulley, I would go with a larger alternator pulley, that is if your engine is going fast enough to keep the alternator spinning, at the minimum rpm needed to produce the voltage needed to keep the battery charged, turn it to slow and your battery will go dead, or your alternator will over heat and die a sad death.
a higher output alternator will have larger winding, and should be over all heavier duty, so it will be a little extra spinning mass, but it shouldn't ever be over worked, I don't think it will be any easier to spin, but it might last longer.
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