A structural element of a body shell, which includes a firewall, two front steel fins and two steel side rails, is mounted by: a) providing a structural element made of aluminium alloy and including, a top, and an inner side edge opposite an outer side edge; b) placing the structural element adjacent to the firewall, while the outer side edge is connected together with one of the front fins and the inner side edge is connected with one of the corresponding side rails. In step a), an outer side mounting part made of steel is also provided and secured to the outer side edge.