early 90's dodge would be great as stated before, but the rest of the truck is junk- trans (auto or manual), rear axle, body, interior. all junk.
i had a 91 6bt/g360 with 4.10s and 35's. awesome tow rig after an nv4500 swap, and overhauled d70-u.
but not as comfy as a gm truck. interior rattled bad, the cabs crack behind the fenders and rattle/squeak, it had rust in the floor. which wouldnt be too bad, but im in oregon.
i have towed a couple times with my current v30. tbi 350/465/3.73. i had 1200lbs in the bed, and an empty car trailer. averaged 12mpg at 70 mph. i also towed a nova on a bigger trailer, average mileage was close to the same, but man was that engine workin hard.
my stepdad has a 94 k30 drw, with a built tbi 350/4l80e combo, and it does better, but man it works hard to pick up speed on the freeway when loaded.
tbi 454's are boat anchors, gutless fuel suckers, but they tow way better than a 350. a 6.5td will keep up with them towing. mileage would be a little better also. but when it needs an engine overhaul, that 454 is going to be a crap load cheaper. but i would not say that either would last longer than the other. i worked on motorhomes, and from what i have seen, 454s just dont last when being used to haul heavy.
i would look for a 96'and newer. a vortec 454 makes alot more power, and would be plenty reliable. i wouldnt rule out a 6.5td, but keep repair cost in the back of your mind, especially if you dont do your own on 6.5.