General engineering question - sheetmetal welding in real life

Welding an entire seam or just in certain spots to hold it together? If you run an entire seam MIG welding puts out too much heat. It will warp thin material and it will stay that way after welding and could warp worse while cooling. TIG welding is better for thin material since it concentrates the heat right at the weld and doesn't propegate far.
If you only need it welded in spots then it could be done with a MIG.

My experience with welding light gauge sheet metal, leans me towards spot or resistance welding the joint unless there is a need for the weld to be continuous.

It should be a complete seam, the total lenght of the box. I'll contact some welders and see what they say. thanks!

Regardless of welding method, structural steel below 3mm thickness is not recommended to weld more than 20mm in length `couse weld produce high amount of stress in welding core and you risk to put in jeopardy flatness of surrounding sheet metal parts due heat exchange in material...try to use soldering with flame torch and soldering silver (lower heat exposure)...or if is not possible, try to make some tools to keep flat elements flat...either way, TIG is better solution...:)
hope it helps...enjoy:)

@Kevin
Spot welding is good solution too...gaps you can fill with some of fillers available on market today...after it, sanding paper, paint... and voala...good as new :)

A full length weld on thin material can be done with Mig, it just has to be done in short sections at a time so heat doesn't build up. Even when done this way it's faster than Tig. Tig will be the higher quality and better looking.

Use older method with acethyene welding but if material is stainless steel or similar use only TIG !