Towing a 911 backwards is ill advised, especially with the engine installed. The rear of a 911 with the engine installed should be over the trailer axles. I have towed 911s backwards over short distances (50 miles or less.) Heavy duty trucks can stand a lot of weight on the tongue but it makes more sense to have the load on the axles. The auto manufacturers and race teams spend fortunes working on aerodynamics, which also helps in towing. If you don't believe me, please see this YouTube video.
https://www.youtube.com/watch?v=JeEEC5eVNC