I have the same thing on the back of the Expanda but minus the spare wheel as it is mounted under
I did not strengthen the bar and it has been along a heap of terrible roads and all is good.
I have seen a bloke with a tool box mounted on his that carries his 2.6 kva generator and 20lt of fuel and he did not do any strengthening
he has done the Gibb River Road and no signs of falling off, when I asked him about whether he was worried about that he replied that he spoke to a fabricator about the weight and they said it would handle it no probs.