i actually beg to differ on the term 'spot welding' as what is usually done to 'strengthen' the chassis most of the time are tack welds. these beads of welds are placed along sills and other integral parts of the chassis structure to stiffen the chassis up.
in my opinion, tack welds don't last and don't penetrate fully because a tack weld is usually done to position the workpiece BEFORE the final weld (ie. butt welds, stitch welds, etc.).
spot welding requires either use of a spot welding clamp/tool which can cost about RM3.5k and usually no one uses these here...so, the next best thing would be to do 'plug welds' or 'puddle welds'. essentially you drill a hole through one piece of the sheet metal/panel you're working on and then proceed to fill up the hole with weld. hence the name plug/puddle welding.