Another trick is to drill holes and weld through the hole. Often with practice you end up with the weld being about flat when done. Or, what I did often when I was restoring my Bronco, get the body part secured in place with self tapping sheet metal screws, then remove a screw and weld through the hole. Looks like a spot weld. And the metal is cooling while you are removing the next screw. You don't want to build up too much heat for fear of warping the metal. Then once you have the part secured in place, use a tube of body sealant and run a bead to seal all the gaps. You can buy a caulking gun size tube of body sealer. If you do the sheet metal screw method, you will have some weld build up on the back side, but you don't see that side. Drilling a hole in one body part and welding through to the other part means no mound on the back side.
Use a weld through primer if overlapping metal so your welds work good.