[POWERSTROKE]

EEEWWWwww. The 5.9 and it's later Big Sister 6.7 are really good engines, Hard to find one with many miles left for reasonable money. Company I used to work for leased a FreightLiner FL70 with a series 50 engine and Allison trans. Thing leaked oil something terrible, vibrated so bad the driver had to crank the door window down and steady the mirror year round to back into the dock. He could Not get out of that clunker fast enough.
After 4 years my new boss asked my opinion on which truck to lease, another FL70 or a Navistor 4400 with DT466E, Allison, Alcoa wheels, air ride suspension, air ride seat, air conditioning, AM/FM/CD player, it rode better, handled loads better, my suggestion of lower numerical rear gears to increase economical cruising speed helped save time, improved mpg, and driver liked the truck so much he took all his breaks in the truck listening to his CD's enjoying the heater or AC. The Navistor ran over 190,000 miles in 4 years and looked almost like new, chrome and aluminum needed a little polishing. The FreightShaker only got 170,000 in 4 years, and looked about 10-12 years old. The leaking oil blowing all over the chassis did NOT preserve it. Was funny, the Only time the FL70 was loaded even close to it's rated gcwr the leaves in the left rear spring broke, dropped the box on top of the drive tires and ground it to a quick stop, driver was just barely able to get the truck off the road enough to let traffic by, had to tow it backwards to the repair shop, transfer the hot freight to a different truck and deliver it. The Navistor only ever needed normal scheduled maintenance.
I doubt a DT466 will fit where your 351, 401, or 6V-71 could fit, but a 7.3L V-8 might, either Ford PowerStroke or Navistar T444E, very reliable 210 hp.