First, Sonny and Cher had his-and-her Mustangs. Then hey, hey, there were the Monkees and their freaked-out GTO thingy. ZZ Top had their Eliminator. And now, The Spice Girls have their own ride as well, the cleverly entitled Spicemobile, thanks to Mr. Kustom-with-a-K himself, George Barris. For the five saucy, sexy, nationalistic British pop sensations, Barris ripped the roof off an all-American Cadillac Seville (not even a righthand-drive one to coincide with the Seville's new push in the UK), stretched it, and transformed the innocent STS into a dual-cowl phaeton. The Union Jack is painted across the hood, trunk, and sides, and the seats are colored to match the girls' hair. No, really. Our only question: Now that Ginger's gone, who's driving?…