As previously mentioned, try reaching out to the artist to update the design to match the new cover dimensions. Otherwise, it's challenging to provide advice on your own since we have no idea how complex the design is. From what I can gather, it's only the width that needs to be updated, this may be easy or very difficult, depending on the design. In some cases, you can utilize automated options in the latest versions of Photoshop: select the empty area and ensure that a small portion of the selection touches the design, then use the Generate feature.
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