As per lynn's comments, if you are welding bits back in and it is being attached to the armour plate, it will need to be done with stainless, for cutting you can use the thin slitting discs on a grinder you can get them off ebay by the truck load for buttons.... they get worn away fast, but as they are so cheap it does not matter, plus the thin profile of the blade makes a neat job... i dont think the track guards themselves are BP steel. I prefer to weld with a Mig, you could run some lovely seams in there
Horsa did a fantastic job of replacing his rear shelf and track guards. What are you restoring at the minute ?