The first gen Cordoba and the Magnum were weight reduced to become the second gen Cordoba and Mirada (our J bodies). Chrysler stuck with the B body chassis for those first gen cars, which seemed logical in 1975 when the first Cordobas came out. it wasn't so logical two years later when GM started shrinking their "big" cars (Impala/Caprice, Delta 88/98, etc.), and the Chevelles/Lemans a year later.