Before mounting the ribs to the spar, all the bolts in the spar need to be removed - they come installed on the pre-made spars. They are then reattached to the spar and rib.  Some of them near the inboard end of the spar need to be reversed to clear the fuel tank rear baffle - as can be seen in the second picture.

Then it is time to start riveting and bolting the ribs to the spar.