After Effects - Green Screen - Subject Was Filmed Using Different Lens Than Background Image
You can use CC_lense to change the appearance of your background so the focal length matches your plate more accurately.
If you have more time (and money) to spare, you can use software like PFtrack to follow the visual lines in both images to estimate the focal length. This would give you the opportunity to accurately distort the image so the focal-lenghts match on a more accurate scale.