I haven't been to Namibia, so have no experience of how close you can get to the game (can you go off the road / tracks etc?).
I do have a feeling that 300mm / 340mm will probably not be long enough (I have a feeling it is all fairly open with little bush, like the Kgalagadi Transfrontier Park), but with the D800 that may not be a problem as you may be able to make significant crops? I have only been to Botswana and find 300mm to be an ok focal length, but it tends to be quite bushy with few areas of open plains. Where it is open, 500mm can feel short!
My personal choice between your two options would be to buy a 1.7x and maybe pick up a Kenko 2x TC too (the Kenko Pro 300 DGX 2x TC can be bought from One Stop Digital for approx £120). It obviously depends on your personal taste for IQ, and how much the D800 shows up any lens weaknesses (probably a lot?), but in good light on my 40D and 70-200 f2.8, I can get useable images with the 2x TC, which may be better than heavy cropping? :shrug:
How do you feel about renting lenses? IF you can rent something like a Sigma 50-500 OS / 150-500 OS or even the 120-300 f2.8 OS (coupled with TC's), or if budget allows, the Nikon 200-400 then that may be a better option than buying the TC's or a 300mm prime.
It really depends on your budget and what IQ you would be happy with for your shots.
Sorry, that is probably not that helpful overall!