Theoretically,spot welding should be the correct term to use if you want chasis strengthening but you can also use seam weld to describe it in layman terms. this is because the actualy seam welding, is another process of welding usually found in automation welding such as shock absorber. usually race prep/rally cars are spot welded all along the car body where there's joining of 2 different metal piece.
if you're a DIY person and have some cash, time and have a space car to drive, yes you can actually spot weld along the joint. and yes, you must use a MIG welding set coupled with CO2 as shielding gas. you CANNOT use gas welding or commonly known as fire welding whereby you burn LPG and heat up the metal piece u wanna weld and fill in with a brazing rod. gas welding will take you years to complete a car body but MIG welding will take you 3-4 days at best, 2 week at most.
i work in the welding line and one my customer who buys machine from me does chasis strengthening and i've seen him done that. very neat and just by one glance, you'll see your car looks like a polka dotted styled car. but obviously thats before he spray paint it. apparently it was for merdeka endurance race last year. anyway, JFA motorsport is the place if you ask me. i think most car tuning shop should be able do spot welding for you, but make sure the shop is reputable, or else if the welding is bad and rust occurs, you rather not wanna take corners at 80kmh!!