err..., lap welds are all over that car, and done by GM
I end up trimming the pans to where they fit the best, and lay them in to trace around them . Then, pull out the new skin and trim the old floor to 1/4" lap. That 1/4" makes a heat sink when welding, plus it gives support when you need to persuade some of the new pans edges to fit tight.
If you think you are up to the frustrations of butt welding an entire floor half...go for it
But I want to see the results