Is 12 mpg attainable

[cam.man67]

So here's the dilemma I'm faced with: my '79 is my winter driver, and in preparation, I've been driving the truck around for the past few days regularly. Good Lord, is this going to be an expensive winter...

See, I've never gotten better than 8.5 mpg... last fill up was an appalling 4.5 (I think I have a fuel leak somewhere, but that's not the point of this topic). Anyways, my financial situation this winter is not conducive to sub-10mpg economy (I'm getting married in April)...I need to do something different.

And so the question: is 12mpg feasible, given the following combination.

-Olds 350, Quadrajet
-TH400
-mud terrains out back
-12-bolt, 3.73 open

I like this truck a lot, and I really need to get some better fuel mileage out of it. Part of this is that I don't want to dump a bunch of parts at it in hopes of better fuel eco. I want to focus primarily on tuning, timing, etc. Is this possible?