Did you call them or just check the catalog?
I looked them up on their respective web pages. And unless I'm overlooking something major, they appear to be exactly what you're looking for, and not just skins or partials. Pretty much the same deal you used on the '65 Chevelle.
On the Year One website, I took the following route to find them:
Year One Home-> GTO-> Body-> Quarter Panels-Full-> Part number FP566LH (driver side) & FP566RH (passenger side).
Or you can buy part number FP566LHRH and get both the driver & passenger side for a very slight discount (of only a few dollars).