Scott, I have to say you have the cutest helper EVER, OMG she is darling, what a little sweetie!
On the floor, I would need to see more of where it needs to be replaced, the holes I see up there in the front are no where near any bracing or floor supports to speak of so being a unibody means nothing, just replace the rusted areas, no big deal. It doesn't have to be perfect you will never see it again after you put the carpet back down.
If it's rusted over a floor support it still is no big deal, drill the welds out and remove it and weld in a new piece.
I have a 59 Rambler too, by mine is the lowly American not a big and fancy Super Cross Country like Kaitlyn gets to work on.
On my floor it wasn't as bad with only small rust holes so I did a quicky sand blast, epoxy primer then fiberglass job over it.
By the way, one trick to locating all the rust is to clean that floor all up then when it's a little dark put a drop light under the car.