Get a long spot weld drill bits in 6mm and 8mm Diameter. the short ones require and angled drill in some locations due to the bubble shape which makes more mess and more likely to break through or not cut the whole spot out.
Hole punch where you want to drill first
If you watch you'll see when you finish cutting through the upper panel, change in colour (rust) and feel/sense it once you get used to it.
If you do drill through hold a piece of brass behind the hole and fill with weld. the weld wont stick to the brass and the brass acts as a mandrel and heat soak...it gets hot, wear gloves. I have a selection of brass pieces. Old cupboard handles, mushroom shaped, big brass padlocks, flat plate/sheet etc
I find it easier to plug holes than try and repaid mangled steel after the cold chisel has been used. Id rather drill it out with a bigger bit than smack the bejesus out of it trying to cold cut.
Take your time and be patient and it will work out OK.
JP