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I'm not sure how much this will apply in your situation, but I noticed with my 1986 F250 diesel that warmup time while driving is a killer on the MPGs. With the E4OD transmission and 3.08 gears, my truck will pass 24 MPG on a good day (long distance @ 55 mph), but for short stop and go trips of less than 6 miles in 30 mph speed zones, the MPGs can drop to half that depending on the weight of my foot.
Now, the 6.9L diesel is roughly twice as much iron as a 302 and produces lower EGTs under light load, so that dramatically increases the warmup time to begin with. Generally 6 miles is what it takes just to reach operating temperature and that doesn't include the transmission or differential (differential seems to take the longest). In the dead of winter it will not reach operating temperature just from idling. I've been told that synthetic gear oil in the differential will make a big difference in the critical warmup period but have yet to try this myself.
What helped in my case is to remove the radiator fan in the winter when warmup time is even longer. For short trips under 6 miles, it netted me a good 2 MPG on average. I'm not advising to run your truck without the cooling fan, but I can see how an electric fan setup will help. I considered doing that with mine but never found the time yet.
Unfortunately, the 302 isn't the most economical engine out there, especially for a fullsize truck. My other truck is also an 86 thats motivated by a LPG powered 302 (roller cam). Best it ever netted was about 13 MPG with average closer to 10 (that would translate to about 13 MPG on gasoline). I would also be weary of lowering your gears with a 302, since they are a mid reving engine that doesn't breath well below 2000 RPM (peak torque is at 2400 if I remember right). Even with my diesel (peak torque @ 1400), the 3.08s are no asset in lower speed zones with lots of starts and stops.
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