This is not a job I have carried out on a Punto, but I have replaced the box on a Peugeot 205 single handed. I supported the box on a trolley jack to take the weight and slung a couple of straps under the box to enable me to move/wiggle it. I do remember that the b*gger would not line up, then all of a sudden it was there. By the time I finished, my forearms were a mass of bruises.
Be careful when adjusting the box that it is still supported by the trolley jack. It is too easy to let it rotate and slip off.
Good luck and keep us informed how you get on.