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Your shifter is probably just worn out. Volunteer Vette (forum sponsor) sells all the individual pieces for the muncie shifter. Also, you can still buy new shifters. The shifter is easy to rebuild by yourself, so you can just buy all the individual pieces from Volunteer and renew the one you have.
I pulled the shifter out of my 68, and the shift mechanism was hidden inside a big ball of grease and sand. Over the years, the sand had noticeably worn the pieces.
Before you take the shifter apart, take some pictures of it and maybe make some drawings. I didn't and to get it back together I had to get another Muncie shifter to look at to get mine back together!
Just about all the vendors have the detent plate and parts for auto shift cars. Figure out the parts you need and break out your credit card. really not that expensive either.