What I learned about the shifter is that is not like either the SS Camero I used to have, or like the Vette. If this is so, then nothing on the market that I am aware of would work.
As explained to me, it is a more European type design with two rods going into the transmission. It was speculated, by a guy that had changed motors and played around with his V that the slop is due to the rubber bushings.
The other problem is you can't get to the top of the transmission like you could with my SS Camero. On that car I put in a B&M ripper sifter and it was a big improvement, but all you had to do was get to the top of the transmission by taking off the tunnel cover.
If you drop your transmission down a bit on the V from underneath you will be able to see the bushings. I am hoping that someone will duplicate them using polyurethane and that will firm things up.