Sound like a reasonable plan. Getting to 30 mpg is everone's goal but you must remember you're driving a brick. I own a 90 f250 which is the same body as your's. It isn't very aerodynamic. If you don't have all you're parts yet you might want to think on a Ford ZF S5-42 transmission and use a Cummins SB Ford adapter/flywheel set up. However, the Dodge set up may be cheaper in the long run. The 2.83 gear should be plenty high enough. Wouldn't plan on any heavy towing with that. What you want to do is calculate your engine speed based on your mph. Should be somewhere between 1800-2000 rpm. Too low and you're lugging the engine and too high it uses more fuel. Although 30 mpg isn't impossible, I would guess mid to upper 20's is more realistic. Much will depend on you driving habits and type of road.
One item many people overlook is the economics of their plan. I've been driving diesels for 33 yrs. If I were buying a new vehicle today I'd buy gas. You pay a huge premium for the engine and the fuel is $.40 more expensive. You have a Ford 300-6 which is a great motor. With a few mods you might get another couple mpg's out of it. Those things have been known to go 300,000 miles or more without a rebuild. Got to remember those were even used in dump trucks. Sound kinda like a diesel. If your engine is shot then I'd consider the diesel. If not that would be a tough decision. Remember if you get to 30 mpg you'll only be saving about $.02 per mile. Takes a lot of miles to pay for a conversion.
