If I remember right, the 3-speed cable is pretty long. It comes out of the cowl, runs along the subframe and then makes a big loop toward the outside of the car so that it enters the tranny straight on. Which hole are you using in the cowl? If you are using the Muncie hole, you may want to change to the 3-speed hole. The cable routing is different and may help with your problem.
I believe (but am not sure) that all Camaro bodies were manufactured with the base 3-speed hole. Muncie cars got an extra hole. The 3-speed hole is located below the steering column to the left. You can see the location in the assembly manual.