A donor is the way to go, only easy way I see to get all the brackets. Once you have all the brackets, it doesn't seem like that hard of a swap, especially if the package tray doesn't need to be cut. Obviously it won't be 100% factory accurate, but if you're doing a conversion it won't be 100% anyway because it won't be on the fender tag or build sheet or whatever.
And I figured out another possibility for why the package tray was cut back so far- it does add quite a bit of vertical clearance to the pass through (several inches at least). Maybe Ma Mopar was trying to make something specific fit in the trunk/pass through area?