what transmission side cover are you using ? Is it a interlock by chance ? What year is transmission ? 70 and up is interlock. Do you have brass shift forks or steel ? what shifter handle are you using ?.
crap I forgot to ask did you pop the cover off the bottom of the shifter and lube it ? When I put my super shifter in the valiant I had similar problems. I popped off the cover and it was bone dry. shot it with some spray lithium grease and presto fixed 75% of issues. other 25% was factory bench seat shifter handle. swapped it out for a transfer case handle and no problems since