I emailed the SAPS Central Firearms Registry about this issue. Their reply was:

"The Firearms Control Act, (Act 60/2000), does not specify what the minimum requirements for the length of a barrel must be, nor that the firearm must be fitted with optical sights. I am not familiar with any written document or instruction which regulates barrel lengths. However, the applicant must motivate the suitability of the firearm he/ she intend to use and for what kind of hunting he will use the firearm. He/ she will follow the normal procedure for the import of the firearm."

That being said, we all know that what should happen isn't always what does happen. I would strongly recommend using the services of Gracy Travel or similar company, taking the advice of your PH and having letters confirming the intended use of the gun being specifically for hunting.