If the rust through is not too large you can splice in a new piece of 16 or 14 gauge. Since this area is also covered by your florr covering you might be able to do an overlay of new material but that is a corner cutting method. The only patch panels I have seen for that area go from the toe board seam back to the front of teh seat. Nothing says you have to use the whole panel though. Be aware that your forward cab mounts are in that specific area also. They tend to trap the moisture which causes the rot in the first place. They may need replacing or patching also.