Are you able to move the shifter in the side to side position to operate the opposite arm? If not disconnect the arm or arms off the trans and try moving the shifter through the shift pattern. If everything moves smoothly in the shifter then the problem has to be in the transmission itself. Possible the shifting fork inside the trans could be bent locking gears together. Maybe the short arm on the trans which connects the long arm of the shifter is in the wrong position, please recheck this after checking the shifter. Eliminate the shifter issue first then move onto the trans. Just because it's new doesn't make it right. Many new part have caused me a lot of grief. Good luck.