[Longhorn Man]

I don't think I have ever heard of an old school carb'd small block getting 20 MPG... not in a truck with the aerodynamics of a barn.
My specs are nothing special... 125xxx mile old 402 (after pulling the valve covers and vfinding litterally not one bit of sludge, I wonder it it has been rebuilt at one point), with a later model (mystery) quadrajet with no working choke, TH400, 3.54 gears (this is a 3/4 ton) with a brand new HEI and plugs. I think the MAIN reason i was able to do that, my throttle was messed up. The cable kept falling off the bracket, and that would leave me with somewhere between 1/4 to 1/2 throttle. on flat land, i was only able to get 80 - 82 MPH... did get a gravity assisted 95 MPH... was shootin for the century mark, but had to lift due to a hair pin curve at the bottom of the hill.
In my normal (fixed throttle) mix of city/highway, I've been getting 10 - 12 MPG. I'm not babying it, but I'm not pushing it real hard either.
Personally, I would say the best bang for the buck you can do for MPG, would to be swapping out the rear axle. (easier and cheaper than just a gear swap) I would stick with the tranny you have, and locate a 3.07 gear rear axle. They were standard equipment on any small block/automatic truck 67 - 72. (TH trannys... I can't remember what came with the powerglide trucks) Again, I'm talking C/10 trucks, not 20 series or K series. This will put your 60 MPH RPM at about 2500 - 2600 RPM... Which doesn't sound like much... but it is a HUGE drop in reality.
This would be very close to a 3.73 gear with a TH700R4 tranny as far as cruise RPMs... and that would set you back $2000+ ... even if you were careful.
Some will tell you that a 3.07 is a total dog off the line... but a bone stock 350 will fry the tires for a 1/2 block with these gears. It would be a noticeable decrease in off the line performance, but it'll still tug hard. Really only an issue if you are towing/hauling at or above your truck's limits, or if you race it.
A junk yard rear axle should be no more than 200 bucks, and I would HOPE it would be more like 75 - 125 bucks. Put your rear brakes on the other axle (assuming they are worth keeping) and all is gravy.