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Unless you are a professional body man with access to a fabrication shop I would consider this plastic Impala as a Parts Car only and look for a better pile of sheet metal in the form of a car body from the desert southwest to transfer the interior and trim parts (as well as a 12 bolt if it has one) from your car into a better one as interior and plastic parts are destroyed by the desert sun that also preserves the sheet metal from rust.
It sounds counter productive, but patch panels are expensive (as are floor pans and full quarters; if AMD is even currently pressing them, which I don't think they are). Because these big boats are not as popular as a 1955-'57 Chevy shoe box or a 1967-'69 Camaro there isn't a complete body stamped out of brand new sheet metal ready to spot weld together to make a new car. You usually have to scavenge sheet metal off of a wrecked car of similar year and model to restore one properly. Your cheapest solution is to start off with the best body you can find (the parts you can not buy new) and ad interior, trim, power train and accessories as time and budget permit.