Or, you could just do an 18 speed, with 2.47, 2.65, 2.79, or 2.90 rears, and run in direct (16th) at 60 mph for max mpg like Matt said. In 16th, with 2.79 and low pro 22.5's, at 60 mph you would be turning just higher than 1400. For something like the DD15, I would probably go with 2.65 or 2.64 (depending on axle make). With 2.65 and low pro in direct, the rpm would be roughly 1375. I just ordered a glider with 18 speed and 2.79 rears. Just match up the rear ratio to the ideal operating range of whatever engine you choose while running in direct on the tranny. An 18 will give you another gear up if you feel the need to move along quicker, plus you will have all the splits into the basement for max pulling. And the 18 has deeper reduction and better spreads than something like a 13, so running a tall ratio will not kill you. Make sure and spec high torque clutch and drive train. If the engine has, say, an 1850 torque rating, spec a 2050 driveline.
I don't agree with Jess on this one. Actually, running in overdrive steals hp from the rear axle. Running in direct, all the available hp and torque is making it to the rear wheels. You actually need MORE hp and torque to run in overdrive than in direct with the same load, terrain, etc. That will cost you more fuel. Jess is right though.... you have to drive it correctly. But that is true for any engine, tranny, rear end combination.
I am not sure why something like an 18 running in 16th (direct) with tall rears causes so many people's brains to lock up. It isn't that big of a deal. Going back to pickups and autos, the manufacturers recommend towing in direct drive only. It is the same principle applied to big trucks when you do something like an 18 tied to a tall rear end.