Hey Seth. So the body work that would need to be done is purely cosmetic when using the conversion brackets that are offered on the website.
Here is a brief overview of the holes that are left when converting from late model bumpers to early: There are holes in the front fenders and the rear quarter panel that are left from the accordion side mounting points, as well as cutouts in the front nose where the late model brackets are inserted (the bracket kit provides the sheet metal to close up these cutouts around the conversion brackets), also there are holes which span across the lower part of the rear panel, I think there is roughly 8-10 screws which are used to mount the rubber flap which covers the gap from the rear panel to the the inside lip of the late model bumper. The rear bracket mounting holes are covered by the rear bumper. Another option, if body filling isn't one, is putting plastic grommets which can be found at your local hardware store to plug each hole and paint them to match the color of the car. This option is more of a "quick fix". Please email me at firstname.lastname@example.org if you have any other questions. Hope this clears some things up!