Jimmy, i would not worry to much about not getting on the trackside of the fence, as you are a lot closer to the action with motorcross than normal circuit racing were you are next to the armco barriers.
The photos in these albums were all taken in public areas but of course it could be a little different at your local circuits, on the corners you were ok with the 70-200 but for head on shoots the 300mm was better
http://www.gforceimages.net/gallery/...s.php?album=42
http://www.gforceimages.net/gallery/...ls.php?album=9
Basically get down to a meeting and see how it goes lol.