If your entire front floor isn't rotted out, then you will want to trim the new panels to only replace what you need. For the front floors, they sell a right and left floor panel that goes from the front of the seat riser to where it joins the toeboard. They also sell right and left floor panels that are combined with the lower halves of the toeboard. They also sell complete toeboards.
The front of the cab is supported by reinforcements underneath which are spot welded to the floor pans and the rocker panels. So if you are replacing that area of the floor, you will need to support the cab.
