Do you know if the detent in your cabin shifter box is intact?
It centers the shifter in the 2-3 gate.
If the turnbuckle is loosened, you should be able to feel it settle in. If you use wedges to center the shifter, try and be sure you're in the detent.
The transmission also has detents. Again, with the turnbuckle loosened, you should be able to manually get it to settle into its 2-3 detent.
Now tighten the turnbuckle. Both the cabin shifter box and the transmission should now settled into the same detent position and not resisting each other.
To center the shifter fore and aft, loosen the shift shaft U-joint pinch bolt at the transmission. It must be loose enough to slide back and forth on the splines.
Manually position the stub sticking out of the transmission to its center position. You'll feel where it wants to settle in. Slide the u-joint on the splines to center the shifter and tighten the pinch bolt.
In fact, tighten all the shift shaft u-joint pinch bolts as tight as you can. Every problem I've had with shifter adjustment has been the u-joints slipping on the splines.