Old post here in our forum:
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=1049452&page=1

Continued into this forum (notice the comment and picture of fork wear and distance between forks):
https://vccachat.org/ubbthreads.php/ubb/printthread/Board/11/main/49666/type/thread.html

Also another possible solution here:
https://forums.aaca.org/topic/319659-1950-chevy-3100-pickup-transmission-linkage-hang-up/

I would add to check for play in the transmission shift-arm shaft bushings.
Also note that the upper and lower box housing is made of "pot metal" and can wear out (fabricate bushings?).

It a nutshell, everything that moves and everything that makes contact needs to be checked for wear.
The problem is there is no specification to determine how much wear is bad and there are no definite pointers on where the wear areas are.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."