If your powerglide will not auto shift at part throttle the vacuum modulator is not getting vacuum or its not connected internally. If the shift/kick down cable is not used the trans has no way of adjusting the hyd line pressure on the clutches at WOT. will tend to burn up the clutches. The "kick down" will not work either.
On a stock valve body they both need to be hooked up and functional. Go see the trans tech that built the transmission. Do not drive at WOT until you get it sorted out.
a delayed up shift at part throttle is a valve body issue or a vacuum modulator issue.
The cam in the motor and the motor itself has nothing to do with it.