I don't know what kind of rig you have, but JC Whitney (and other places) often carry floorboards ready to weld in! That can be the easiest option, fits nicely, save a bunch of fabrication time, etc.
That being said, we usually take various flat metal stock and cut out the rusted piece and BRAZE in the new pieces. By braze, I am talking about using an acetylene setup and using brass rods. The temps you are using is alot lower...dad has been rebuilding cars this way for well...nearly 50 years now. This works out really well if you are doing "Pretty" stuff...the nice panels/brazing them in, etc...
However...if you own a MIG setup....practice until you are comfortable with welding sheet metal...that is probably the FUTURE of doing this kind of work. Dad/my way is the old style. I think with the higher MIG heat you have to do VERY small sections at a time, let it cool down, etc.
As they said, you will have the two pieces overlap each other...not a tough job, but would practice with scrap metal first!! When I bought my MIG (ReadyWelder) I went out and bought $20 worth of scrap iron and having been playing since then trying to get good.
I am NOT good by any far fetch...but "good enough" to weld stuff up on our farm on the tractor, that kind of thing. Ugly/booger welds but they hold.
Oh yeah, if OBXFISHERMAN still has them, he had some really good pics where he welded in new panels in his XJ! You might try PM'g him.
Sam