I went on my first African safari four years ago and since then have been back once or twice a year, so you could say I am addicted! Let me also say I am envious of your trip, you are in for a treat.
On every safari I have been on I have upgraded my equipment, the reason being that much of the good action occurs early in the morning, in the evening or at some distance. You need long lenses yes but you also need fast lenses, the faster the better. I also have a 100-400 L and agree that it is a fine lens for the money, but you will struggle with it in less than good light, especially so with a 1.4 t/c! It is 5.6 at 400, plus a stop if using a t/c, which is going to be no good in lowish light with a moving subject.
I now use a 500 f4 prime on one body and the 100-400 on another, and that covers most situations. you need the reach as often good action (hunting etc if you are lucky) will be going on some way from the vehicle, but you will also see some good stuff up close'ish, so you need that zoom as well. Obviously if you don't have a long fast prime lens then I am sure you will get some good shots anyway.
Hope that helps and have a great time!
ps - don't forget the bean bag and leave the tripod at home.
pps - I took this picture with the 100-400 and I don't think it came out bad. http://www.photoportfolios.net/portfolio/p...i=PAULE&u=11610