I hope you've considered all that extra weight in exactly the wrong place, behind the rear axle.
The twin wheels would be 50kg, the bar another 50 maybe ? Then the Tvan ball weight of around 200 and if you extend the tow shank to allow clearance, that's adding more than the 200 normal ball weight. We are talking about 350kgs on the rear of that car and that is bad stuff. The pay load of a 300 series is only about 600kgs so you don't have a lot left after all that gets added.
The twin wheels would be 50kg, the bar another 50 maybe ? Then the Tvan ball weight of around 200 and if you extend the tow shank to allow clearance, that's adding more than the 200 normal ball weight. We are talking about 350kgs on the rear of that car and that is bad stuff. The pay load of a 300 series is only about 600kgs so you don't have a lot left after all that gets added.