this is a topic that comes up every 2 or 3 months so there are lots of recommendations if is you search the old posts
i'm not sure how much better the 400 5.6 is than the 300 4 +1.4, but my 300 +1.4 is just marginally better than my 100-400. on my 20d they both with produce excellent 13x19 prints (but bigger than this would not stand close inspection)
i'm sure from the many comments on the 400 that it's better at 5.6 than the 300 + 1.4 or 100-400 - i get noticebly better results at f7.1 or f8 even if i have to go to higher ISO. IS is marginally useful as usually you need reasonably fast shutter speed to stop action
all 3 of these lenses will give good results handheld on a 20 or 30d or even Rebel, but they are not long enough or sharp enough on a full frame camera as your primary long lens on safari
to take advantage of the 5d or 1DS resolution, you need a 300 2.8 with 2x, a 400DO with 1.4X or a 500 to significantly improve on the results of the shorter, lighter, cheaper lenses on a crop frame camera. And, unless you have a vehicle pretty much to yourself (and an assistant?) , the big glass is going to be hard to manage
so my recommendation is to buy a crop-frame camera to use with your 300+1.4 and take care of the big, close animals with the 5d and a 100-200 f4 with a 24-105 for really close stuff and scenery -- in general, swapping lenses and extenders just doesn't work -- things happen too fast and the sensor gets to dirty
my last recommendation is a telephoto flash for fill during the day and for images of spotlighted animals at night - this makes a significant improvement