IF you can find a mid-late 90s Ford with the 7.3L, you'll definitely see better MPG than with the gas 460. Without knowing the numbers, I'd think significantly better...maybe 15-16mpg in mixed driving with the diesel, and 10-11mpg with the 460?
Now that the gap between diesel and gasoline prices has closed somewhat, that could equal a not-so-insignificant yearly savings in fuel costs.
Your biggest obstacle with an older truck like that may prove not to be powertrain but rather accessories; things like power window motors, windshield wiper motors, AC compressors, fan clutches, etc. sometimes wave the white flag after 15+ years of use and countless heat up/cool down cycles.
My thoughts on older vehicles has always been: if you're the type that's inclined to keep it in tip-top shape, don't mind spending a Saturday here-or-there replacing this or that, and have a little bit of a repair budget socked away, older, high-mileage vehicles can be an incredible bargain.
If you're in the camp that wants to just hop in and go and will need to take it to a shop for every little issue, keeping an older vehicle on the road can cost every bit what a monthly payment on something newer would.
Anyhow, good luck in your search and of course keep us posted as to what you come across...