Karl,
Do you still have the registration number (from the original UK number plate) from when it was imported, or any paperwork with it on?
There is a form you can fill out and send to the licensing authorities over here (DVLA) requesting all of the records they hold for the vehicle.
It's not always complete, and I'm not sure if they would be willing to post to SA and I'm not sure if it would work for a vehicle permanently exported, but it might be worth a try.
The form is the V888,
HERE.
In fact, looking at it again, they ask for proof of address etc. so might not work for you.
If you want, I can try to call DVLA this week some time to ask if the form would work for you for overseas owners.
Of course, if it's a '68 and if it was driven to SA in the late 60s, there is unlikely to be any useful info held by DVLA

In which case, Phillip is your best bet to at least find the build date.
Cheers,
Robin.