TF 904 Auto slipping 2nd to 3rd when manual shifting @ higher revs

That. Cable/linkage that connects to the lever, which int turn contacts a valve in the valve body, to increase the line pressure of the trans. Proper adjustment is when carb at wot, the lever fully moved. So when you're at wot, the trans has the pressure it needs to function correctly and SURVIVE!!

If they are Mopar guys I would stop taking advise from them.

If they are Ford / Chevy guys, well you get what you pay for.