Not to be dense, but you say "To get more reach than my current equipment," and mention that you already have a 400mm, when your post is about a 300mm lens? I understand you could make it 600mm with a 2x, but still....

I would think a TC, like the 1.4x III, would be a better fit for your needs on a budget. My first recommendation to you, though, would be to buy a clean used copy of the 50D, which would give you an EFL of 640mm with the lens you've already got, and at f5.6, it would still AF. Also, you'd still have access to AFMA. This will only set you back $600-700 at most, and would give you more pixels on target than you have with the 5DII.

Also, wanted to point out the obvious, which is that this would also provide you a backup body, just in case.