How long has the trans sat?
I sold a standard 833 A body trans a while back, the buyer wanted to look inside, no problem pulled the shift cover off.
the tops of the gears had surface rust on them from sitting for a long time.
The trans would not shift correctly, he bought it, and texed me a couple days later, said he drained the fluid, soaked the trans in diesal fuel, and it turned over fine, and shifted.
You may have a corrosion issue.
