There is a shelf below the battery that goes back behind and under the firewall. The shelf on the passenger side is the same.
On my 63, there are styrofoam wedges wedged in just behind the plane of the firewall.
Are these stock?
What are they for?
Is it OK to remove them?

They're present on all 4 of my 65s as well. IIRC when I parted out a 66 Dart GT, they were there as well.

They are splash shields. If you remove them you will see into the fenderwell, and if you go through a puddle without them, a lot of water will get past them(I wouldn't doubt that a little gets by already, but not enough to worry about).

You could probably fabricate something out of sheet metal that would look better. That is if anybody looks that far down into the engine bay for it to matter.

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My cars are all gone, my new affliction is my Jeep Cherokee. I have lifted it, put some armor on it, and preparing for new gears and lockers. After that, bumpers, on-board air and a winch.

I was wondering about that myself. Thanks.