Having recently come back from Botswana I can tell you my mistakes and what I will take next time.
I took an Ergorest with a Manfrotto 222 Grip Action Ball head Click here for the Ergorest Website
which if I had done my homework properly would have realised it was no good at all. Fortunately as I had paid for my own safari truck for the duration I was able to get it clamped onto one of the rollbars and slide it from left to right depending on what I was trying to view.
One of the many birdwatchers that was at the same camp showed me his clamp that was made by velbon (although I haven't found anything like it yet) It was almost like something you would find in a DIY shop to hold sheets of wood to a work bench but with the right size mount on top for a tripod head of some description.
Depending on the size of your lens would depend on the head you need - lots of people I know use a king cobra make by Kirk for big lenses. It does depend on your subjects - are you a bird man, general wildlife? I found that the 300mm lens I took was in the main too big (I'm not a bird man!)
Have you been on safari before? I found Botswana much more intimate with the wildlife than in Kenya or Tanzania so we could get closer and use a smaller lens. I did take some shots of a leopard lit up with a lamp at night with the 300mm handheld which came out very nicely. I just couldn't get the angle I needed from my ergorest and they look pretty good even if I do say so myself.
Feel free to ask any questions - only too happy to help. As my most recent trip to Mexico has been cancelled I am thinking of going back in March/April (if my wife lets me!)