yo will find professional welding shops weld all along edge.this give maximum strength and reduces corrosion issues around such things as a spot weld. however its not unknown to have these spot welded but be aware of lower strength and potential test fail.

You will need to fully weld them to pass roadworthy. Use a mig.

spot weld , every 3 inches you can rent a spot welder if you don't have one , and they are surprisingly cheap they do far less damage to thin metal especially OLD metal than even mig or tig welding

Take it by the body shop on a trailer and ask how it's done.

They can tell you what you need, tell you where to buy it, or do it for you.

Or, you could watch how-to videos on you tube.