Your best option for getting your gun rights restored is by requesting a pardon from the South Carolina Department of Probation, Pardon, and Parole Services.
This is done by making an application through their site at http://www.dppps.sc.gov/ Or you can also contact someone from Pardon Information at(###) ###-####
Once you apply, there will be a hearing scheduled in which you will request your case be pardoned, so that you can have your gun rights restored. Since this offense was over 40 years ago, if you have not had any other incidents with the criminal system since, I see no reason why you won't be approved for the pardon.
The reason for needing to apply for a pardon through this route, as opposed to the federal system, is because currently all federal pardons and gun rights restoration have been on hold for some times due to the fact that Congress failed to alot the necessary funding to the pardon and gun rights restoration division.
This would normally leave your next option as being an application to have your case expunged, which would then restore your gun rights. However, South Carolina does not permit expungements for certain violent felonies. Fortunately, South Carolina does have the pardon and gun rights restoration process in place through the above mentioned department-- and they do in fact grant gun rights restoration for even cases that do not qualify for expungement.