My '77 used to do that if I shifted it too aggressively. /forums/images/graemlins/frown.gif The tip of the shifter gets jammed between the blocks on the shift forks. The fix is pretty easy though!
1. Remove the shift boot.
2. Press down on the dome shaped part right where the shifter enters the tranny
3. While pressing down, turn the domed part counterclockwise 1/4-turn.
4. The shifter should now lift right up out of the tranny.
5. Using a LONG screwdriver, move the shift blocks so that the notches in them line up to form a square. This should be neutral.
6. Insert shifter. The tip should fit into the square formed in previous step.
7. Press down on dome shaped part and rotate clockwise 1/4-turn.
8. Reinstall shift boot.
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