Wrecked the Valiant. Pictures here.

You need to be fighting this money figure they came up with. Classic cars can be "argued" into a different category than some "blue book" market value. One way is to challenge them to come up with a replacement car in comparable shape for that amount. You and I both know it cannot be done

A /6, 4 door '75? Yes, it's clean, but so is my '74 Duster and I bought it for $2k.

You might be able to argue them up some, but 4 doors don't get a whole lot. Usually they're going for less than half of a comparable 2 door.

That looks like a ton of damage. Frame, inner fender, K member, radiator support, etc. Props to you if you decide to fix it, but I would be looking for another car. If you could locate someone with a project at the rolling chassis phase you could swap all of your usable parts over. Not saying that car can't be fixed, but the cost of the fix will definitely exceed the cost of a straight 73+ A-body.