A904...does it need a rebuild?

try adjusting the throttle pressure linkage...aka the kickdown linkage..you want to try to shorten the upper rod....

if it is too far out it can keep the trans from shifting into 3rd...or you have to let off the gas for it to shift...

a simple test is to disconnect the linkage at the carb....drive around block..the trans will shift thru the gear really fast at low speed or rpm....if it goes into 3rd...then the linkage is out of adjustment

but dont leave the linkage disconnect...it will burn up the trans