Who is BS'ing who?

First up, on your engine, the 280/480 cam is 238 @ 050 which makes it a poor choice for any kind of mileage. Good for a street machine though. Also, your rear gears (3.23) converter (3000 stall, LOL!) and the tire size work against you.

Secondly, a list of mileage items would look like;

Open air cleaner
smaller carb. 600 on a 360, 500 on a 318
Dual plane intake, AKA OE iron, Performer or LD4B for a 318, LD340 for the 360
Headers help, but dual exhaust @ 2-1/4 for a 318, 2-1/2 for a 360 max size.
Add a "H": pipe.

Add a MSD or equal like ignition and open up your plug gaps to .055 or better if you can.
Vacuum advance distributor!

Electric fan/water pump, less parasitic drag.

Trans;

no stall, stock 2bbl verter
Add a cooler and a deep pan.

Rear gears, HWY.! (LOL) and a reasonable tire size. 26 - 27 inches max height.

Follow those suggestions. My '79 360 in a 3800+lbs. '79 Dodge Magnum got 20 mpg's all day long with similar equipment. The MSD added about 2 mpg's. Tune that carb and then tune it again and again and again. Take note of how it drives going down the road and run it as lean as you can for driving. Richen it up well for part throttle and W.O.T.