Do the Gs500f REAR fairings just fit right in where the gs500e fairings did on the rear or is there modification required?
Some modification is required
You will have to change the side grab rails as they have part of the tail mountings (buy a pair from Flea-Behy or from me).
Here is a photo comparing the tails of the "E" and "F" models. The basic frame is identical. The difference is the tail where the grab handle mounts and the rear tail plastics. The seat mount on the "F" is slightly higher.
The silver is "E" and the black is a cut off "F" tail. You could cut of your tail end and weld on this "F" tail end. I'll probably cut it up for scrap as I know of no one that wants this "F" tail section that was given to me with other parts.
(http://i1206.photobucket.com/albums/bb442/adidasguy/GS500/DSC01667a.jpg)