Hi Mark,
I've had a long battle against the dreaded rust, and I've got it beat now!
zillions of hours later, 2 welders and litres of Metal Ready and POR15!
hints.. Ken McLean did a tech article on sill replacement, available in the technical section of the club site, would be very useful to read first.
In my experience the rust you see (especially in the sill area) is the cliche "tip of the iceberg" if you have rust flakes, dirt and sand, I would guess there is more underneath there. Once again the tech article should help you remove and replace the outer sills.
The floor is pretty easy, just place your replacement section over the area, mark it out and cut to about 1 - 1.5 inches inside the line, re-fit and trim more accurately as you go. Less is best... measure twice cut once.
An overlap of about 0.5 - 1 inch is desirable for a mono floor (to retain strength). Stitch weld in place, on both sides then seam seal it up.
I use weld thru coating to ensure the welded area doesnt rust out again prematurely.
I'll see what I have in the way of photos, most of them aren't scanned so bear with me.
Make your plates as accurate as possible, less gap the better, spread your heat and don't rush it.
The best quality welding gear you can get your hands on too.. I prefer MIG but better panels are achieved using gas. Also get yourself a good Grinder.. Harko can elaborate more on this.
As Stinky suggests practice practice practice.....
Good luck
Cheers
Ed