26 mpg in a big full size truck with a cummins will never happen, except on cases with extreme modification. the 12v is probably the way to go, you'll have a tough time finding anything else that easy to make work, let alone with the performance. not sure when the cummins switched to EFI.

powerstroke is a great choice too, but it's got some electronics to deal with. i used to have a big stock E350 powered by a 7.3 that got 22.9 mpg once in the middle of winter, so it's capable.

remember these big diesels will not only be huge, but weigh close to a thousand pounds, and need some figuring to make fit.

__________________
1981 300SD - 283,000 KM's at purchase, 360k+ now. Engine replaced at 311k. 16" CLK wheels, w126 gen II cosmetic upgrades, late w126 leather interior. RIP. Parted and gone due to fire

1987 300SDL - 243K miles and counting. In winter hibernation!

2001 Ford F250 Super Duty "Platinum Edition" Lariat 4x4 7.3L turbo diesel, 295k+ miles, various mods for reliability and performance.