I have a 98' 12v with a 47re. I've heard so much about how you have to rbuild every hundred thousand or so miles. I don't want to up the power more than like 50hp and only tow maybe 9000 lbs a couple times a year. All I want is for the transmission to last a long time so I do t have to constantly rebuild it. Any suggestions on what I should do?

 

Well, if you keep it serviced (fluid changes, filter, band adjustments) they do last a lot longer than 100 k miles.
What does work nicely on improving the reliability is:

A master kit with added frictions in the direct clutch and overdrive.
Add a better front band piston, and all new seals.
A billet accumulator piston is also nice to have.
Also a higher rate lever for the front band. You will have to adjust the front band a little more often!

Should be good for over 200 k miles if maintained.

I got my rebuild stuff from Tim Barber, a really good guy to order from in my experience.

Something to consider when upping the power:
You will need to let the tranny know that there is more power, because you will have less throttle movement for the same amount of power. So the tranny does want to shift extremely mushy at a high power level, and that kills it.
Therefore, on a 12V, I prefer to use a GSK to get some RPM out of the engine, and then I adjust the AFC to a smoke-free position. This means that you have to really press the pedal to get the power.
This might not "feel" like a powerful tune, but since the transmission now "knows what's up" and also has enough rpm to build good fluid pressure to apply the clutches the 47 RE can and will stand quite high power levels for a long time.
(Built one for a friend with a 12V, 180 k miles of trailer towing on a twinned, cammed etc engine, 90% trailer towing with a lot of mountains.- Truck was lost to an inattentive driver after the 180 k miles. Did tear into the transmission, wear was indicating it still had some life left, case was cracked by crash)