I'm thinking adjust your motion track for position and rotation but NOT scale. Or, scale it only a tiny bit, by applying the tracking data to a null and parenting your BG to the null with only slight scaling.
Farther objects in the distance (i.e. mountains) tend to remain roughly the same size (not accounting for perspective objects, trees, the road etc.) I guess if your footage was shot out the back window of a vehicle it might be ok as-is, so I'm assuming you're constructing the read window shot?
If so, build it with 3D layers - I've found that 2D never pulls it off, especially with the side window stuff. You need parallax to make it work. I built an entire desert complete with joshua trees in trapcode Particular once just so I could have side window scrolling, and even then it was merely passable.
Andrew Kramer has a tutorial on this very task in his Serious FX and Compositing DVD.
http://store.creativecow.net/p/52/serious_effects__compositing
Brian
