Here are a few forums to look at. The DTR has a great tech section, and it's free. You have what Dodge guys refer to as a First Gen truck, 1972-1993.
I have a 93 D350 dually, Cummins 5 speed, it gets 21 on the highway empty. Mileage goes down a bit the more in town driving that you do.
If yours is an 89, it won't have the intercooler, one could be added for a bit more power.
Watch the front end, it's loaded pretty heavy with the Cummins up front. keep an eye on the frame where the steering box bolts on, even 1/2 tons have been known to crack around the bolt holes. A little quality welding and a bit of plate reinforcement fixed mine. Borgeson joints to replace the factory
pot coupler on the steering column are a good improvement on these trucks too. Good luck with it!