Trans problem, need advice

B'cuda said
I have a 69 340 A727 with a full manual reverse valve body. The converter is 340 high stall. The cam is the old Direct Connection 474/280?
The problem is after the trans gets hot, it slips on take off and hardly has reverse. In both cases, alittle rpm and it drops into gear. It pulls great and the 1-2 shift is solid, chirps the tires, the 2-3 shift is instant with no lag. I run a separate stacked plate tranny cooler with fan.
My first thought is bands or trans but could it be the converter?
If it is the bands, is there a way to tighten them to get anything?
Any advice will be appreciated

Rev and it drops in gear is almost always the clutch pack piston seals.
Fluid is leaking past them and hot fluid being thinner leaks past them quicker, making it slip when hot.
It's possible it's something else, but usually that's the symptom of worn rubber seals.
A plugged up filter could also do it.
It isn't the converter.