Trans problem, need advice

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