Back to work.
With the arch cut off and repositioned in its new location and marked, its time to get serious. The excess metal needs to be removed. Since I'm going to flange the metal and weld the arch back on top of it, we need to trim off enough metal so the flanging tool will put the edge of the flange just outside of the arch, so the weld can be right at the edge of the flange and the arch. If properly placed, the weld should fill the dip between the two pieces of steel, and little or no filler will be needed. I measure from my line at the edge of the arch cut, and draw another line 3/8" under the arch piece, all around the arch piece. (I use the arch piece to draw this line as well.) When I cut the excess off, I will cut right on the line, I do this with a pair of aviation snips, they can give you a very accurate cut. Aviation snips are available is a right hand cut, a left hand cut, and a center cut. I have the right and the left hand cutters. They are also available with a low angle hand position and a raised hand position. For me, the high hand position is easier on the skin on my fingers, I get less cuts on my hands from them.
Pic 1, The lines. The two blue lines are the correct lines, the upper line is the desired location of the welding edge and the step of the flange. The lower blue line is the cut line.
Pic 2, The aviation high hand right hand cutters.
Pic 3, The aviation cutters in operation.
Pic 4, The final cut. Before that cut lies was drawn, I had already cleaned up the edge of the arch piece, and smoothed out the gaged edges for a smooth flowing welding edge and gotten to shiny steel on both sides of the arch piece.