Outstanding! The first guess and you have won the point!
Let's go through the pairs:
1-D BMW 700 by Giovanni Michelotti & Fiat Argenta by Edgardo Michelotti
2-F Fiat Uno by Giorgetto Giugiaro & Alfa-Romeo Scighera by Fabrizio Giugiaro
3-B Fiat 1100 Mille Miglia by Ugo Zagato & Alfa-Romeo SZ by Elio Zagato
4-E Fiat 1500 Cabriolet Ghia by Felice Mario Boano & Fiat 850 Coupé by Gian Paolo Boano
5-C Chrysler 300 C by Virgil Exner & Simca Special Ghia by Virgil Exner Jr.
6-A Fiat 600 Coupé by Domenico Giannini & OTAS Tigre by Franco Giannini
Actually these people aren't exactly the designer, but they are the one who "did" somehow the car.