Here's my 'stock vs stock' story I've told here a few times over the years.....

Back about '90 I had a '76 Club Cab, 2 wheel drive, 8ft bed, 1/2 ton with a 2bbl 360 in it. No matter what I did, that 360 would give me no more than 12 MPG. Put any kind of weight in the bed, or drive aggressively, I could EASILY drop it down to 8-9 MPG.

I had a '77 440 out of a cop car that I swapped in. I never went in the engine, internally it was just as it was in the cop car. The engine was originally Lean Burn controlled, so I swapped on an earlier TQ and (stock) electronic ignition. I used headers only because I couldn't find manifolds.

Behind the 440, I used a 727 out of a '77 truck that had a tighter convertor than the cop spec hi-stall unit. Out back I had the 3.23 9 1/4 rear out of the cop car.

In day to day driving, I'd get no less than 12 MPG. If I drove sanely, I could hit 14 around town. But, I was addicted to the 4bbl wail of the TQ so, it was usually around 12....

I drove the truck from MA out to Phoenix, AZ with most of my worldly possessions in back when I went to school out there, and actually got 16 almost 17 MPG on the interstate. If I had to guess, I had about 800 lbs worth of stuff in the back.

After the 440 swap, it NEVER dipped into the single digits as the 360 so easily did and the truck was easier and way more FUN to drive (ever throw 18" ft of truck sideways on a twisty dirt road? ) .

Now, I understand even a cop spec 440 , especially a '77, isn't the most powerful engine out there, but the difference between it and the 360 was night and day.