Floor repair options

[LittleDually]

First off I am fairly new to restoration so a lot of this I am learning as I go.

Now that the disclaimer is out of the way here's what I am working with. I just pulled up the old vinyl floor mat and insulation etc. in order to put down some sound deading and paint the cab. I discovered, on the passenger side, that water must have gotten in the cab some how. The matting was wet and there was some severe pitting of the metal. Thankfully I only found one spot that was rusted all the way through. Now what is the best course of action to repair the floor? Do I need to cut and replace all the pitted area as well as the hole or is just a small patch ok? Is welding the only option at this point?

Here are pics of what I am looking at.