What is this trunk panel called, and where can I find one, please?

As I mentioned in my introduction (link: New member with an interesting story. ), I'm working on a '71 Chrysler VG Valiant from Australia, and amongst a couple other things I'm currently working on, today I had to pull out the gas tank.

When I removed the filler tube(?) that goes from the fill cap to the tank (my apologies if the terms are not correct), I found that someone had previously used a bunch of Bondo, filler, and/or fiberglass in an attempt to hide this little beauty here:

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To be completely honest, this is the only rust I've found on the car ANYWHERE, and the entire underside of the car, including the gas tank, is da** near SPOTLESS. (Weird!)

Anywhoooo, while this car may be a '71 from Australia, from the back the body looks almost exactly like a '68 or '69 Dart to me:

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I have to believe I can find something here in the States that will work, but I don't know whether to look at a '71 vintage panel of some sort, or, given the external similarities between this car and the US Dart, buy a '68 or '69 panel, so can someone please point me in the right direction here? Can I use a '69 patch (or complete) panel, and if so, where can I find one? I'm not about to have something shipped from Oz or NZ, nor do I have the time, as I want (need!) to get this repair done before I can put the tank back-in.

Any and all useful input will be sincerely appreciated!

Thanks.

Jim