Sounds like JeffreyG may have given you the info you really need, which is basically that the 5D3 produces acceptable results at ISO 12.8K (second the Wow!). Which actually isn't very surprising. I'm quite happy with my 5DII at 6400 (with some liberal noise reduction). Now, all you need is a camera and a lens that can stop down to f/4, which it sounds like you have. Take it to the location you plan to shoot and see what shutter speed you can achieve at ISO 12.8. . .or if your camera doesn't go that high, see if you can get half the desired shutter speed at ISO 6400 (you get the idea). Then, as long as 300mm is going to be long enough, you should be good to go.