If your rain gutters are not all rusted out, then your rig deserves a nice little 5.3 LS-1 engine
(a) less weight
(b) better milage
(c) impressive power
The down side of this is that if you ever want to be a puritan and build another 6 cylinder rig---there's just no going back!!!
When I was looking at 5.3s for a possible swap, I was finding mileage reports of around 19-20mpg in platforms that were 7000lbs (my loaded 62 is about 6200), so I would expect better mileage than your 3FE by a long ways. You might also be able to detune the motor with a laptop and appropriate software, but all that is voodoo to me.
5.3s are dirt cheap, and there's a wrecking yard in Oklahoma that specializes in super clean removals with engines that have 10 miles on them (for $1500 last I saw), computer, wiring and accessories.
I would not detune for milage.... he$$ a V-8 is to have fun with. 5.3 are on ebay.... There are some slavage yards that sale "pull-outs" all the time on ebay and will ship to your location...engine+ most parts, + wiring harness +ECM...etc I bought a 5.7 vortec from a place in TN.
If you want to go crazy get a 6.0....one of the newer ones...almost 400 HP. I say to he$$ with MPG and hello to fun.
If you want to save some money in the long run ...buy a GM crate long block and install the cylinder heads and fuel system of your choice.