Are you missing the cable from the servo to the carburetor, or just the attachment at the carburetor?
If its just the carburetor attachment I would look online and at the parts store for the various pieces and parts in the carburetor parts section. In most cases the attachment at the carburetor is just a simple post linkage that is similar to your throttle cable connection. Rostra and others also sell aftermarket cruise controls, and may have an adapter/fitting for the GM linkage.
Bruce