Hi Jo,
I did my rangie which is similar to your disco, 120x60x5mm secrtion, (it was free, 3mm should be fine). couple of tips;
1. cut the original sill along the bottom to leave you an upside down channel, that way you don't need to make new body mounts and it easier to weld.
2. cut out the strengthing ribs inside the sill, you dont need them
3. jack up the box section , if you drill a couple of small holes in the floor , one at the front and the back abovethe sill, you will be able to see if the box section is fully up into the sill. You can also puddle weld the sill and box together using these holes.
good luck.. :D