[Caddylackn]

You are not going to do much better than a tuned in quadrajet for gas mileage. The small primaries make for better fuel atomization at cruise. Rebuild yours if you need to, don't buy a rebuilt one. Your factree carb is jetted and calibrated for your motor.

For gas mileage, I would swap to a factree HEI, with the mechanical advance recurved to get full advance in under 3,000 rpms, and an adjustable vacuum advance to eliminate pinging at partial throttle. Use 3.07 or 2.73 gearing.

The other tip is aerodynamics. Remove the large dumbo mirrors if you have them and replace with smaller mirrors. A bed cover will give you better mpg. Some on here have added a suburban lower front spoiler, which doesn't look bad on our trucks. High teens mpg is possible with our trucks with a stock 350. Drive sensibly.

If you have higher gearing and a turbo 350, consider swapping to a 700r4 tranny.
Tires make a big difference. Mudders and aggressive off road treads give poor mpg. If your tires are loud at highway speeds, then they are sucking mpg from the higher rolling resistance.