Assuming you have the sm420 with granny low transmission. Two things could be the culprit. First the end of the shift arm could be worn and second, the three-four shifter yoke could be worn too.
You can remove the top of the spare trans by removing the 8 bolts, make sure both transmissions are in neutral. then replace the entire assembly to the transmission you are using. Just align the sliders on the main shaft with the yokes of the top loader and it will slip right in.You possibly might need to slide the reverse/first gear a bit forward or backward to fit it all in nicely. Easy/peasy

Last edited by sstock; 06/20/2018 11:56 AM.
1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration