The 3.4 may give you a little better mileage. It isn't going to be amazing. I get 18-19 summer time city, and 20-21 hwy summer, with a 3.4 supercharged in a 99 runner, which is a lot heavier than your truck. However, maintenance and mileage play a large part in that. Mine only has 230 on the clock, and has been babied by an old lady most of its life. Oil and other preventative stuff has been done like clockwork, so the motor is tight and runs very well.
What you find for a swap may not be as good, and I've heard of 3.4's doing no better than 16 even in lightweight trucks.
That said, the swap will be easy, and has been done 1 billion times, so there is a lot of help and aftermarket support out there.
Baby Beast 2- 1999 4Runner SR5
Baby Beast -1987 4Runner SR5-Gone, but not forgotten