gear box upgrade

Sure you can. Have no idea how it'd effect fuel mileage. Make the truck a lot more drivable in my opinion.

 

You can do almost anything but it depends on how much $$ you want to spend. Best bet is to find a wrecked truck with a 13 speed.

In Brighton Co they have a 2006 Volvo ISX with a 13 speed. Looks like it was laid down on the passenger side but trans and motor look ok. They want 9k or best offer for the entire truck.
It's on Ebay.

 

You can stick whatever you want in there. Will more than likely need to have the driveshaft length changed though.

 

You can but it would be very expensive. Probably $5-6k for the transmission with no core and then the drive line I'm pretty sure will be too long so you'd have to have a new one made.

 

I doubt a 13 will improve fuel mpg. Without knowing what the 10 speed is what 13 you want to use its an impossible question to give an answer too.

Think of it this way. Just for the sake of saying a 13 would deliver .3 mpg better than a 10. Just for round figures let's say it's a 6k swap how long till you actually see money in your pocket?

 

More moving parts, more things that can break lol.
If youre trying to save fuel get a new set of gears, slow down, add skirts etc.
While a 13 speed may grant you additional overdrive, you get some parasitic loss from running in overdrive.
I miss driving a 13speed, but even running oil field we mainly only used the overdrive if empty or bobtailing. Not really any mountains to climb in North Texas lol.