I see where your are coming from now and it looks like you've already found your pot of gold.
Isn't the transmission, shifter, rods and arms stock parts used in the MY6 option?
If so, they would be in the Chevrolet master parts lists. My book only goes to 1957.

https://www.ebay.com/itm/164931980502?hash=item2666
https://67-72chevytrucks.com/vboard/showthread.php?t=588774
https://www.gmsquarebody.com/threads/new-process-a833-4speed-w-od-my6-mm7.3422/


"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."