[JQmile]

In 1989, Dodge diesels came with a 3.07 rear gear...if you bump the timing and swap in a gear vendors or overdrive trans out of a 91-93 model, you should be able to hit 30+. You'll also have about 250-300 hp with the turn of a screw, and be able to tow 10K. You could also get a later 91-93 truck and swap in the earlier rearend. Might be easier than a trans swap. There is a guy over on some diesel forums that gets 33 mpg in a 1-ton dually with this setup driving 65+mph. If you slow it down to 55mph and use a lighter truck, I think 40 is possible.

Edit: I just saw your goal of 30 city....that's one thing this type of swap won't do....you'll probably get 17-20mpg if you drive with a light foot. But it does work well on the highway.