64-66 cuda fold down seat into 63-66 dart

Just be aware that the fold-down back seat of the first generation Barracuda weighs 70 lbs. Remember, when folded down, it's part of the cargo compartment floor and so the seat back is reinforced with a rigid, ribbed metal stamping that adds significantly to the weight of the seat.
Also, the X brace on a conventional two door should not be removed, as it adds quite a bit of strength/rigidity to the body. Early Barracudas have a whole bunch of extra bracing around the back window to make up for the lack of a package tray and that X brace. That's one reason - along with the heavy seat and the huge rear window - that a Barracuda outweighs an otherwise equally equipped Valiant by somewhere in the neighborhood of 150 lbs.