I had to weld two small plates on my footwells about 10cmx10cm, although that seems like its been welded before and not treated afterwards. Its pretty scary how thin and flimsy the metal is around that area. Once you have finished welding, get some red oxide paint and cover everything with it inside and out.

Ive peeled back all the tape on my inner sills and filled them all with Bilt Hamber anti corrosion treatment. I bought 2 canisters and filled everywhere with the stuff, chassis legs /rails sills etc.

It sat on axle stands and bled for a day or two until it set. Good stuff though. Give it a go once you have finished!!

http://www.bilthamber.com/dynaxs50.html