1967 Dart trunk area floor pan question

Hey FABO. I’ve got my rear end out to swap to an 8 3/4, so I decided to pull my gas tank and weld some patches into my trunk area while I’m at it. My ‘67 Dart is surprisingly solid through out, I’m pretty lucky. However the worst rust on the whole car seems to be in the trunk.

I can easily find a whole new trunk pan, but there is a metal panel just to the front of the vehicle that it attaches to. That’s rusted pretty badly and I don’t even know what that section is called. It seems to be a filler panel between the trunk floor pan and the “under rear seat floor” (that the shocks mount to). A picture is worth a thousand words, so let me post a few.
Photo 1: under the car with the gas tank removed.
Photo 2: close up of the damage, you can see where the two other pans attach to it.
Photo 3: damage on the front (towards engine) passenger side of the trunk.
Photo 4: damage on the front driver side of the trunk. Note it’s forward of the hole for the fuel filler neck.
Photo 5: replacement trunk pan. Notice this pan stops short of the area with most of the rust damage.

Any advice would be helpful. Thanks!

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