You will need:
1400x60x120 box sextion, around 3mm is fine. Chop out the side of the sill and the bottom, then using a jack, lift the box section into place, it will be a perfect fit.
It's best to slope the ends of the boxes and seal them with a plate. Now seamweld all the way round and done!
Very easy, it takes about 3 hours per side to do. Taking the doors off makes things easier.