I have a 1966 LeMans that had the same problem. I don't think you'll find another package tray structure for yours unless you cut one out from a junkyard and weld it in yours. I left mine in place, ground as much rust out as I could. I treated it with Rust Mort and put a fiberglass overlay on it using three sheets of mat and resin. After it set up, I recut the speaker holes. The car has been finished for a while and this worked very well. Looks good with the vinyl covered tray with a series of little holes over the speakers. I don't know if there is enough metal left on yours. It worked for me and the tray cover solves any cosmetic issues.
After thought: It helped that the rear window was out.