Well with the MK1 and MK2 fuselage, The MK1 one is a fuel tank, so it is heavier and weaker. You can get the MK1 normal structural component in the Structural tab, which is lighter and I think stronger. The Normal mass (full) of the MK1 fuel tank is 1.1 t, and the full mass of the structural fuselage is 0.4t. It has an impact tolerance of 70 m/s, and costs 550.