Your truck will get better MPG with the 3.55 than the sub 4 something gears you want. Nothing at all wrong with running a 318 magnum 1800 RPM. You will have less vacume in the intake for the pistons to pull down against. It will also make less friction inside the motor. You will be takeing a step backwards. If the truck would get better MPG with the lower gears the factory would have put them in it.

2600 RPM is way beyond the "sweet spot" for MPG(I am guessing you mean tourque peak, but even that don't make sense because you don't cruise at WOT so that tourque peak means nothing at cruise).