[POWERSTROKE]

BILL - I haven't had a DEF diesel but I drove a '96 F250 with 7.3L PSD for 23-24 years, and 304,000 miles. I gave it to my Son, he traded his Cummins for his mid-life crisis, a new '18 Mustang GT. He fixed a lot of things that needed done to that truck, and it's completely reliable again, at 315,000 miles.
But I bought a gasoline pickup. Hard to justify a diesel when I only drive 2500-3000 miles a year now.
I'm surprised your 637 V-8 got 5-7 mpg. I drove a late '70's IH FleetStar single axle tractor 478 V8 and straight 5 spd, ran 62-65 mph between the Mississippi River and Des Moines 5 nights a week and got between 3-4 mpg grossing about that same 45,000-50,000. 13--1/2 ft tall 45 ft trailer caught a LOT of wind. The '81 or '82 F700 Ford tractor I drove for 6 months before the FleetStar had a 8.2L Detroit Fuel Pincher diesel, I got an honest 7-8 mpg pulling the exact same trailer hauling the exact same loads. But every couple months it needed a tune-up, troublesome fuel system. Drove a single axle dump truck two summers in college, C65 Chevy's with 366 V8 and 5+2. I really can't say what they got for mpg, no gallon counter on the gas barrel at work, but if I ran hard enough hauling rock out of the quarry I had to put some gas in for the last load. I was the only hired help summer of '74, I put a LOT of miles on a brand new C65 that summer. It had much better rear gears for road work than the '67 C65 366, 5+2 did it replaced, but the '67 spanked the '74 bad plowing snow.
The thing I see guys have the most problem with on old gas powered trucks is keeping the brakes working reliably. Second biggest problem seems to be finding rebuilt carbs and fuel pumps.
When I was a kid Dad drove for a couple livestock haulers, think they all had GMC Big V6 powered trucks. Livestock hauling was the last segment of trucking to start using diesel tractors. Lots of old farmers thought the diesel exhaust hurt their livestock, tainted the meat just before butchering.