How about starting with France ratifying the treaty of Paris and the EDC gets founded? If the merger treaty happens more or less in time in the late 60s integration of the inner six would be noticeably beyond a mere economic integration. I mean the mission statement even back than was to further integration into a unified Europe, but a lot of states mainly saw the economic side when they joined and did not really believe that the founding members truly meant for the Ever Closer Union to happen. With a degree of actual military integration that misconception is dead in the water. The EFTA will be the choice for nations only economically interested in Europe while the inner circle is more committed. Today the (much smaller) core might not be a full federation, but much closer integrated and without the member states that joined without actually wanting real integration on all levels.