All that means in terms of sheet metal is that you need to be sure and spread out your base properly...
I do not recommend welding inside a car... and for sure not from below.... lots of other ( and usually stronger ) solutions.. even if you are willing to pay a professional welder to set your car on fire.

Hint:
Aircraft welds are able to be x-rayed for quality when done and in future tuneups... at regular intervals ...

The rest of the plane is RIVETED TOGETHER.... because that is stronger, cheaper and lasts longer.