They use the wings for fuel storage in either design, so the rear mounted engines are balanced, and not causing any drag. In fact, having the engines mounted to the rear on the body causes less drag, as the rear mount, sucks away more of the barrier layer, more efficiently, than the wing mounted engines does. Moreover the rear mounted close to the body engines are safer, if one engine goes out, as the plane still moves forward, and level. Depending on conditions, with wing mounted engines, one going out, can cause lateral spin of the fuselage. Almost always fatal.