Depends on what you want to spend and how bullet proof you want to make it. Driven correctly and given the right care the trans is capable of lasting well into the 200,000 plus range. However, at the weight you are looking at it will definitely shorten its life and accent the shortcomings.
It is hard to justify $3k to $4k for a transmission in an older vehicle unless you intend to keep it and use it for a while. The other question you need to ask is what will it cost if it goes out in Valentine Nebraska and you have to have it towed to the nearest place that will touch it.
With power upgrades and your weight I would definitely suggest a TC and VB upgrade. That will address quite a few of the issues. The rest, unfortunately, will require a complete rebuild and you may want to consider upgrading to a newer heavier unit.