You could use any of the companion books, as the showdown rules, with some exceptions, are the same as the SW rules that are used when PCs, NPCs and Extras battle against each other in battlemap scale. All the gear, weapons and NPCs from the books could be used. You do have to make some modifications such as aligning a vehicle's Top Speed with the appropriate Showdown speed category. I say any of the companion books, but in fact some are more compatible than others - the Weird Wars series in particular.
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