the argument about "the CTD has a bigger tank so its gonna hurt more when you fill up" is COMPLETELY ludicrous. If a vehicle gets 25 mpg vs one that gets 14 mpg, you're gonna pay less in fuel...end of story. It might cost more per fillup, but you fill up wayyyy less often. Look at the range. His current truck probably has a range of less than 300 miles. the CTD will have a range of over 600 miles.
As for going with gears. If you're looking to save money on gas, modding is retarded. Yes, you'll get better mileage with gears, but its gonna take a LONG time to pay off your $500 investment in gears.
If that CTD is only $14,000 i'd get it in a heartbeat. Yes its got some miles on it (and no, thats not just "broken in". Its still well used, but far from retirement), but still a great truck. You could easily get close to $14k for your truck as well.
I'd get the CTD -- FINANCIALLY it makes more sense.