I think it were mine is get a pressure gauge and see how much pressure it has on accumulator port. If it's holding 90-105lbs at wide open throttle then is adjust pressure so it's 120ish.
This is easy to do and will make the clutches hold better.
Mine had 270k clutches could have been coaxed for another year or so but my second gear band was gone. I finally ran put of adjustment on it and lost second. My tongue converter was also done.
If truck is still driving and shifting through the years well bump g pressure will make the all the clutches hold better I to run cooler and last longer.
The transgo shift lit will be in 150lbs ballpark set up on mild side. It wouldn't surprise me if the old seals can't hold that pressure.
I'd really do lots of research on doing shift kit on a high mileage tranny.
You may also consider running tractor transmission fluid. Or 50/50