The mk2 bench is much bigger (taller and wider) with different fixings than the mk1. If you manage to get it under the hooks then there's no way of securing the back without mods so it kinda just sits there unsecured. It is generally recommended to recover the mk1 bench with the mk2 foam (if needed) and material, please note that this will require some cutting, apparently isn't too difficult though (famous last words)

Hope this helps