Here you can see where the new piece has been welded into the headlamp pocket. The thin line you see is where the new piece meets the old. The welding was done from the back so that you don't have to do any grinding on the front side. Notice how the Mig provides super penetration when you are a good welder. Notice also that I am not doing the welding and that is for a good reason.
The headlamp pocket was cut out of the old panel and then measure, measure, measure. The new panel was cut out and the headlamp pocket was fitted to it.