How do you fix this?

Unfortunately over the years, I purchased two cars that had been swapped over to 4-speeds and when the PO cut out the floor section they did it from inside with a torch so the same section of frame was cut out with the floor section. I just went to the local junk yard, found a donor car, and had them cut the same frame section out, but a little bigger for me to trim to fit my car. I trimmed it, welded it in, and never had a problem after.