Please don't put that stop leak stuff in your transmission. Get it fixed right first or pay to fix the sludge and further damage from the stop-leak.
Hard parts and clutched might be difficult to find, but seals are readily available.
https://skidim.com/drivetrain/transmissions/pcm-40-series-parts.html
Agree with the above on the damper plate. You might get relief by rasing the idle speed a smidge. Spec calls for 600 RPM IN GEAR and UNDERWAY
https://skidim.com/damper-plate-pcm-...n-ford-gm.html
Start with any Dexron ATF and bumping the idle a bit and see where you end up. The leak needs to be addressed through.