FORGET the 4 speed. "If you have money to burn" you could consider an OVERDRIVE 4 speed, but again, you won't pay for the mpg with the money you spend converting.
Those cars ROUTINELY used to get "touching" 20mpg. 12 is dismally LOW. You should EASILY get 15-17, anyway, out of a 318
The factories ALL used to use the same strategy with carbs. Very small primaries, and big secondaries. Quadrajet for most GM, Thermoquad for Mopars and I don't remember WTF the Ferd Autolite carb used to be -- I didn't like 'em.
That carb may not be "the best," and it might be jetted fat.
Numba Wone:
Run a compression / leakdown test and determine if the engine is actually in good shape. If possible, check cam timing.
Numba Teu:
CHECK carefully TDC accuracy, timing, mechanical and vacuum advance. GET the ignition "up to" factory standards
Numba Tharee:
Try to find a carb with small secondaries. Holley used to make "economy" carbs, but if you can find one used that isn't all hacked up, that's great. You might be able to gain a lot by reworking the one you have
Numba Fow-war:
Check tire pressures, critically look at what you have for tires, and things of that nature. Take a GOOD look at your own driving habits. "Feather" the throttle when leaving a stop. The suggestion earlier about reducing the accelerator pump shot is good, but learn to drive without moving the pedal very much.
Now, listen up. I REALIZE gas "ain't what it used to be," and I realize that alcohol diluted gas does not deliver. "Back then" I put a low miles 71 340 in my 70 sixpack Roadrunner. This was MUCH heavier than our Darts, plus it had a Dana 60, a heavy, power-eating rear. This car only had 3.54 gear. That 340 would deliver over 17 and many times over 17.5
It had "kneee knocker" add on AC, Thermoquad, and I routinely (was in the Navy then) had 200 lbs of crap or more in the trunk.