I changed mine from an auto to a manual about 3 months ago. RPM transmissions in Indiana did the work. I traded Rodney my entire trans setup, torque tube, shifter, pedals, 3.15 differential. I got an RPM micropolished MN12-Z06 trans, 3.42 differential, Z06 shifter, pedals etc. Including the labor it cost $2500.
You need to really find a shop that is outstanding in this area as there are some pitfalls. These include issues with the wiring harness, reverse lockout. I have read some posts from other members who did it and had problems. Rodney and guys did an outstanding job on mine and plus I got an RPM built transmission all at a reasonable price. I loved the car before but I really enjoy it now.
My advice is make sure you have someone do it that has done it before. Any questions you may want to email firstname.lastname@example.org
I would highly recommend that if nothing else you get your trans from him