[D13]

So I generally try to stay out of these, but..

23mpg is very possible in a square. 200+ hp is easy.
Together? also possible. But.. the cost to do so will be more than the savings on gas, ever.

1981 C20, 250 inline (about 120 hp), no overdrive, 3.43 gears and 235/85LT16.5's, I can make it get 15 mpg at 55, 17 mpg driving slowly everywhere at 48mph max (no kidding. actually did it one week just to prove a point to my dad).

1987 R10, 6.2 diesel, 700R4, 3.08 axle, P235/75R15, gets me 22-23 mpg in mixed driving. If you run it constant 53-58 mph it will return 26+ mpg. At 70mph, 20 mpg should be good. About 145hp with the J code intake.

My 2016 K10 regular cab long bed has a 5.3l, active fuel management, 3.08 gears, topper, and gets 22mpg with way over 300HP on tap. Runs on 4 cylinders at 65mph on flat ground and hauls butt when pushed, even as a 4wd.

Squares have poor aerodynamics (they are a giant barn to air flow). You need to drive 55. Period.
6.2 sips fuel but is gutless. So, swap in the AFM 5.3l drivetrain. Whoa, that's gonna cost about 4 grand.

For my 20000 miles per year, at 20mpg, that's 1000 gallons of gas. At 25mpg that's 800 gallons. So if I save 200 gallons a year times the $2.50 gas is running that's $500 per year. So in 8 years I have made back the cost of the updated drive train.

OF course, this assumes you can keep your foot out of the 5.3, never drive over 55, don't live in the city, don't tow, etc.

Other option is the 4.3 V6 or a 250 straight six, with a turbo set to kick in at 2200 RPM +. Cruising rpm of 1600, need to pull downshift and spool up. Cost about 3 grand to get it set up, using TBI for fueling. Still not cost efficient but cool.

#1 factor in fuel economy is driving behavior, # 2 is weight, #3 is aero, and #4 is engine efficiency. This is because the drive can control the efficient use of the power to manage the weight and the aero.

BTW I currently drive a 4 banger Equinox, 29.8mpg over the last 80,000 miles, can make if get 30+ for a week if I really care to. The 87 and the '17 sit in the barn until I have a specific need for a truck, or just want to kill skeeters.