Welcome from a fellow 4Runner owner!
The 3.4 is an almost bulletproof engine (assuming you're talking about the 5VZ), but I don't know what it would do for you in terms of MPG improvement. Maybe you could put the smaller engine with a turbo (a sortof ecoboost type of setup).
In terms of tires, I would go tallest possible for highway and road trip driving and shorter ones for the offroad stuff.
This site will give you specs including what combos of tires/wheels will fit (like offset and fender height) as well as what the height is in inches.
This site will show you what RPM you will see at certain speeds in each gear and what a different tire height will do for it.
Is there a different final drive that you can swap to lower the RPMs, so they aren't hitting 3k at 70?
If you're looking for better FE, best way is to change your driving habits (this happens continually) and the other best way is to change it from a brick to an aerodynamic, fuel sipping machine. Add an air dam, seal any holes in the front (like fog light openings with no fog lights), add a belly pan, remove roof racks, etc. You can also do a kammback or boat tail for the rear end. You can build a boat tail on a trailer or hitch mount and then it can be removed when you need the hitch.
If you haven't yet, check out t4r.org for 4runner specific questions.