I would definately reinstall the stops on the aftermarket shifter. Then adjust them.
The aftermarket shifter already voids any warranty the trans had.
Hold the shifter in gear, third or fourth will do fine, and tranitioin on and off the throttle. You will feel the shifter move a bit forward and backwards when getting on the throttle. This tells me that there are not stops in the trans.
Tremecs don't like fast shifting very much either. The synchros go away quickly. Remember to use GM synchromesh fluid or Redline ATF not gear
oil in tremecs.