The benefit of my experience with installing a used transmission is “Don’t trust any seal. Replace those that you have reasonable access to”. The same I think applies to your original transmission when these seal/oring leaks start after ~30+ years.
I preemptively replaced the front pump, piston, TC seals, and of course the oil pan gasket in my donor transmission
But then after completing vehicle reassembly and driving usage, other external seal seepages became whack-a-mole, seeping and replacing in series.
The disappointment is that when the donor transmission sat on its pallet, replacement of some of these other externally accessible seals would have been undemanding.
When the B2 cover seal and kickdown lever pivot pin oring and bowden cable oring (first oring didn’t take) began seeping, seen dripping over the lip of the new pan gasket, I erroneously suspected the pan gasket and chased ghost pan gasket leaks for a while.
Here’s a video illustrating most of the leak points.
it’s probably best to just knock ‘em all out, when the opportunity avails itself.
There are a few seepage areas that can just be “sealed up”, as Greg pointed out w/r to the kickdown lever pin, but if/when the trans sits on a pallet, just do the circuit and replace them all.