Keep going.
Pic 1, At some point you loose the clamps, but the process stays the same. Add another tack weld mid way between 2 others as you move around the piece.
Pic 2, As the tack welds get closer together, you need to put the new tacks a little farther apart, you want to keep the heat down as much as you can.
Pic 3, The filler pieces are welded and the high spots on the welds are ground down. You need to be careful grinding, it can make nearly as much heat as the welding does, and car warp the metal just as bad as welding it hot. The arch is clamped into place.
Pic 4, Lots of clamps. Bet you can't guess how I'm going to weld this in place.