I guess im asking if there is a structural difference between spot welding vs a full bead of butt weld.
That's sort of a loaded question in this particular situation. In my opinion, for most backyard hobby guys lacking tons of experience, I would go with the lap on those floors.
I did do floor halves on a 68 Chevelle factory 327 4sp. As I recall, the halves fit pretty good, BUT.. you can't expect any repro panel to fit exactly the same as an OEM mint floor panel cut out of a donor car. That repro one would lead to serious problems when trying to get both the car & panel, trimmed EXACTLY for true butt welding. That's why I said "show me how it comes out". I can't imagine trying for a perfect trimming on a full floor half from front to back. These repro pans may sit a little low or a little high in certain spots when you try to line up the sides of the pan where they curve up. Like I said, I keep fitting, marking & trimming both the body and the pan, until I get a decent fit. I may pull the pan out a dozen times for more tracing & trimming. Then you still need to force certain areas of the new pan to meet those curves.
It won't hurt anything to try to start fitting and trimming for a butt weld, but as you get into it, I think you will see what I mean. The pan will fit terrible before you get the trimming somewhat close to an inch overlap. Go for an "inch overlap trimming" to start with, then see how things line up. If it fits really good, trim to a 1/2" then 1/4" lap. Then you will need to make up your mind to either trim to butt, or stay with a slight overlap. I would never butt, but that's me...I'm old, and I need to spend my time on more important areas...that actually show