Floor Pan Replacement - where to cut?

1965 Dart GT. Replacing the rusted passenger floor pan and need to know the best place to cut it at the top. My plan was to install the new pan all the way to the upper seam. I already cut it to the top and popped the spot welds loose along the frame rails, as shown. However, the top seam doesn't seem to come apart easy, and it is complicated since a splash plate comes to the seam, behind the front wheel (see photo from wheel side). (The strut tied below pan protects the fuel lines while cutting/welding).

For those who have done this job, would it be simpler to cut it short of the seam, a few inches below (above the red line), and make a butt weld there? I can still get a copper spoon up there from below to avoid burn-thru. The metal is sound once the vertical turn starts.

For materials, I have the $55 floor pan shown, that is sold on rockauto for "62-65 Dart, Lancer, Valiant". It is only a rough fit, and will take some bending and beating. I have the cheap $90 wire welder from Harbor Freight I have used on other jobs, including sheet metal. It does sputter and spit, but works. I have the interior stripped, side windows out and the front and rear windows and dash covered with plastic sheet, so a weld splatter doesn't mess up my glass or dash. I will sand-blast and paint the frame rail interiors while I have the opportunity.