Judging by the preponderance of perforations, and the appearance of a light convex bend alongside, but just slightly oblique to the transverse bead in your photograph- the floor is weak, and has permanently deflected under load already.
It needs to have its sub standard structure replaced.
Count your blessings if you can obtain an accurate replacement panel. Do the cutting out of your original yourself so that the seam will be least likely to cause warpage, and be least visibly different from stock when done.
No sense trying to save old bad metal, if you have new identically detailed stock to replace it with, for after its painted it will both work and look as if it were original.
Leave it to a good professional welder to properly tack, and then fill in between these tacks in such a manner as to fully seam weld without creating significant distortion.
If you do all the preparatory cutting to the welders specification, you will probably find his charge to be reasonable, and the result to be the best deal overall.
Just make sure that your welder is both experienced and comfortable with the proposition before giving him the job.