Frank -
I did two comparisons of the Fusion "under the bubble" :
In this picture the front window is under the bubble :
In this picture the rear spoiler is under the bubble :
From the above, I think the "safest" aero thing to do would be to make a shorter spoiler. Get the ztyle of the spoiler, but keep all of the car "under the bubble" for aero.
CarloSW2