As I said January 2014 (when I am able to make my second payment on the lens) and pick up my Gitzo GT5542LS tripod & Jobu BWG-Pro2 gimbal head. I'll have to tolerate the monopod for the snowy owl irruption in November/December 2013. But at least I have the 600mm. I can haul off from the dyke without disturbing the birds.
My Birding Gear: Nikon D300s, TC-20EIII, 70-200mm f/2.8 VRII, AF-S II 600mm f/4 D IF-ED
If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family anatidae on our hands. ~ Douglas Adams
Wow, you're going to have arms like Arnold Schwarzenegger after packing all that around all summer!
I also would certainly suggest just going with the straight 600mm until you get the tripod going. That's plenty of lens. You may also wish to make or buy a large beanbag that you can put on your opened window on your vehicle. I have used one in the past and was most impressed with it. Stability and enough light is the key but you already know more than I about that! Link to the type of beanbag I was mentioning is below.