It really all comes down to a balancing act with many variables to consider. Even a short bed step side weighs about 4,000 pounds empty, with the aerodynamics of a brick, and a built in speed brake above the windshield. 20 mpg isn’t unreachable, but it would be easiest to obtain with an automatic overdrive transmission, optimal rear end gearing and electronic fuel injection on whatever engine is selected. The trade off being the cost of the upgrades compared to the savings in better mileage.
What’s most important is setting up each truck to fit the realistic needs of the owner. Years ago my daily driver was a 1954 Chevy 2 door sedan. With a 235 six it got about 10-12 mpg, and after I swapped in a 283 V8 it jumped up to about 16+. Not sure how much of the improvement was mechanical vs driver input. I was much easier on the V8 because it would climb to speed easily (and in the beginning I was still running the break easy stock transmission and torque tube rear end), without shoving the pedal to the floor all the time like I did with the 235. The 235 in this case was a bad fit for me because I wanted a hot rod and the six was a low rpm torque motor.
My current driver is a 1960 GMC short step with a stock 350, Super T10 4 speed, and 4.10 rear axle. Normally this would be a ridiculous combination for a daily driver, but it’s ideal for me because 90% of the time it’s driven at 35 mph, and rarely goes above 50. Last time I checked it was getting about 16-17 mpg. If it were driven on the freeway the mileage would probably drop near single digits.
Personally I like both the six and the V8 for different reasons. The sixes are simple and easy to work on, make a lot of torque, can last forever with almost no maintenance, and are popular at rod runs because they’re not very common anymore. For around town driving they can be ideal, where theoretical mileage may not be all that important if the truck doesn’t rack up a lot of annual mileage.
A modern V8 spins higher, faster and is much easier to tune to lean running, because so many combinations of parts are available compared to six. A small block V8 is physically smaller with a nearly unlimited amount of inexpensive speed parts available.
Any combination of V8, six, automatic or manual transmission can produce an ideal arrangement if it’s suited to the application. If an automatic transmission is chosen, a lock up converter will lower engine speed. A few years back I swapped in the 4 speed manual replacing a Turbo 350. With no other changes, my engine speed dropped 500 rpm in drive by eliminating the torque converter slippage. An overdrive transmission can also eliminate the need for a rear axle change. A .70 overdrive will change a 3.90 rear end ratio to a 2.73, providing potential savings, but it is important to match the final drive ratio with the power curve of the engine. If the engine is running at a speed below where it makes torque, mileage will suffer.
