[Boog]

16 is within reach but that 305 will have to be running in what I call the sweet spot as far as rpm is concerned. Around 2,000 rpm at the usuall highway cruise speed should net you the best mileage. You can use gear ratio calculators to figure what it's turning now and see how to get it to where it needs to be. www.richmondgear.com has a couple of calculators you can play with. It is a big help.
You say at 65 the tranny hunts for a gear. That's a real good sign the truck is geared too high. My 88 with 305, OD trans and 2.73 rear did the same thing. At 70 it was turning way under 2,000 rpm. Dropped it out of OD and rpm picked up and it pulled much easier. My guess is your's has a 3.08 gear and would benefit from a change to at least a 3.43 if not a 3.73 if you are going to the 255/70/15 tires. Take a look at that richmond gear calc and play around with the numbers and see if you can't better your combo.
Also pm piecesparts about his combo. I think he is reporting in the neighborhood of 20 with his 305.