Its gonna be awesome with that 3.4! Im havin the same troubles that you were with the 22r. It just costs too dang much to rebuild for not much power. But since my truck is my DD and wheeler I can't really give up the mileage the she gets. So it looks like a 4 cyl rebuild. I can't wait to see the new power plant in your truck! Good luck! 
I'm getting pretty much the exact same MPG with my 3.4 that I did with my 20R. But I get where I'm going a hell of a lot faster than I used to.
A 3RZ is a great mix of power and fuel economy if you're concerned with that.
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'80 Pickup, 5VZ-FE engine, A340F transmission, Marlin gear drive adapter, Dual Ultimate transfer cases, 5.29 gears, Detroit rear/Lockrite front locker, 37" Goodyear MTRs, XRC8 winch, custom tube bed and armor
