For longevity:
1) Keep current on transmission fluid/filter changes.
2) Big transmission cooler.
3) Extra capacity transmission pan.
You really don't want a truck that you have to baby and schedule around the outside temperature to pull a big hill. This is just nuts. You want to put in a quality rebuild with heavy duty innards... a "Built" transmission. If you found a transmission guy you can trust I would tell him that you want a rebuilt transmission with upgraded components. Ask him to put an external transmission cooler on when he installs the transmission. I have also heard good things about Jasper transmissions. You should be trouble free for an easy 100K to 150K with a few transmission services along the way (Change fluid and filters).
Alison transmissions are tanks. The cost to put one in your truck is likely to be more than your truck is worth. That does not sound like a good investment to me!
A used transmission is also a bad investment. Like they said above half the cost will be labor... so get a quality rebuilt transmission to start with. Who is to say a used transmission won't crater on you in 10K - 20K? You will always be worrying about the transmission instead of just driving the hill when you get to it. If you just want to get your truck running again so you can sell it a used transmission is probably your cheapest route.
I wouldn't change to anything but a drop in replacement. If you put an electronic transmission in you will just have more things to go wrong and more initial expense. Stay with what you have but upgrade the internals.
How many miles are on your truck?
