It's not impossible But I don't know how much cargo you could carry and still get 40 mpg. I am pondering regearing mine again (89 F150 4.9 2wd) Originally had 4.10's now 3.27's and I'm leaning towards 3.08's, even on the highway at steady 55 the engine wont settle into a steady load so it pulses dipping to 27mpg on accell and soaring to 45+ on the mini coast. Soon I hope to get some weight in it and see what I can get when it is working.
Funny I just saw an old Ford Commercial for early 80's F150 they boasted about 20 city 30 highway mpg.
With how I used to drive
I got 13 on average. still holding down 25 mostly in town.