Looks like a good plan to me. As I said in my first reply, I would use lag screws with washers in oversized holes through your support boards and into the underside of the table. These support boards should be something like 1 1/2 x 2 1/2" with the narrow edge against the table. You can counter sink the lag screws into the support board so the heads don't show. I would suggest a hardwood for the supports rather than fir. Fir has very little strength to fight against any amount of cupping in the table top slab. Something more like Oak, Hickory, or Ash would be much stronger.
A suggestion for the end skirt boards...run them past the side skirt boards by about 1/2". That way when the slabs expand and contract the side skirt boards can move in or out and still look good on your table design. You could even do a fancy profile on the ends of the end skirt boards. You can attach your side skirt boards to the legs in a solid joint, but allow movement for the legs to slide along the end skirt boards by attaching through slotted holes.
Last edited by MNsawyergp; 12-12-2013 at 12:58 PM.