Steve,
The detent mechanism built into the shifter normally causes more trouble than its worth, and is duplicated by a detent mechanism built into the transmission. Most Panteras have had the detent mechanism built into the shifter removed, it results in a much easier shifting transmission. I can shift my trans with 2 fingers holding the shift knob, gently guiding it through the slots of the gate, very litle force is required to shift it.
I would remove the detent mechanism from the shifter, make sure the trunion bearing is clean, lubed and adjusted well; and the grease inside the selector housing on the side of the trans is clean and fresh instead of sticky and gummy. Then determine how the shift lever behaves when pulled left or right. If you pull the lever into the R - 1 or 4 - 5 slots & let go, it should spring back into the 2 - 3 slot. If there is reasonable spring back force (centering force), the springs inside the trannny are OK.
On the other hand if you pull the shifter into the R - 1 or 4 - 5 slots and it just stays there, or the centering force of the springs is weak, then it is possible the two springs on the selector shaft have lost their tension.
your friend on the DIC