Lap joints are for hacks (jmo) and leave a place for future rust.
We have all sorts of factory lap joints in our cars JR.
Just from replacing my floor pan, every side was a lap joint. Some offset ones as well in other key visible areas.
It’s a floor pan, a lap joint won’t hurt. It’ll be seam sealed on both sides (I assume).
Unless your building a concours correct car, even then, lots of factory lap joints to copy.
I will say though, on a visible panel (like when I finally patch my lower front right quarter) a butt weld really helps with keeping a patch panel hidden. Even then it’s no guarantee. I’ve seen butt welds show through even with a mud coating over the weld area.
Various issues like metal thickness and types of sheet metal come in to play in butt welding.
Its important to consider if you can adequately access the back of the panel after the weld to seal and protect the surface.
Like the spot welds I did along the inner rocker panel when I welded in my floor pan. How do you protect the interior of the rocker? (Between the inner and outer rocker). There is a spray sealer you can buy, btw that you spray inside the rocker.
Lots of talk on my part to say that lap welds aren’t a terrible thing. You just need to be able to seal the weld area up sufficiently after you’re done welding.