rusty sills - box section?

got some nice holes in my outer sills so they need repairing, a quick chat with S71 and 'box section' came up. is it as simple as just cutting off the outer sill completely and weld in a length of box section? if so what size should i be looking at? and where could i buy some from near Bournemouth? B&Q? engineering place? or is there a specific type of supplier?

cheers, Mark

I think its 100mm*60mm 3mm wall thickness. there are threads on here nd other sites detailing it.

lr4x4 & Difflock