Nate
If you where to go the route of the diesel I am pretty sure you would need to swap out the rear gearing. Poke around here for a wile there is a lot of drive train info here
http://www.6066gmcguy.org/ As stated by Oldsub most of the old 5spds are not O/Ds but there are some. The link above has all the info you need. For the mileage you are going to want an O/D regardless most of these old trucks are good for only 50 MPH and they are screaming at that. My 64 2 ton gets about 7 mpg at best with a S/B 350. Another idea and the way I think I will go is to use a Gearvender O/D they build one that is good for 30,000 GVW. Split you rear once at the bottom an once again at the top and split all the tranny gears with the Gearvender and you will have 12 gears to work with.
