omg i cant beleave someone would take a tack welded chassis for a spin on public roads what a tit!

as for cutting steel the only way to mark out is a scribe as if you need to work to the mm then anything thicker than .5mm is too fat
cut just off your line then file the last tiny bit back to the line
a tip is if your eyes are bad ir the steel is bright and hard to see the scribe line is to use marking out fluid or marker pen on the area where you need to scribe