If you can get someone to help you, put the car up on a lift and have someone shift through the gears while you watch the rods from below to see if there's any noticable binding, or excessive slop due to worn bushings. Newer Hurst shifters use nylon rod bushings, while older designs used more durable metal ones. Either way they're probably well worn by now and due for a replacement and both styles are readily available. While it's not as good as sending the whole unit out for a professional rebuilding, new bushings and a proper adjustment may get you by for now. Also, keep it well lubed with some lithium grease...or some Amsoil product in your case.
Can you share some more info about your Yenko Camaro with us? Pics, description, selling dealer? I guess we can assume it's a 4 speed. Where ya from Steve?