I would think you could make the 1040. A route I often travel is to take the BA 380 to JNB and then an SAA flight to another Southern Africa location. It usually takes about an hour for the faffing, immigration part, although a hassle. The 380 is also quite fast, at times the departure was up to an hour after scheduled time (causing some handwringing on my part), but the plane made up the time in-flight.