When your Miata was built, many of the seams were only spot-welded in order to speed up production. Seam welding - also known as stitch welding - involves improving these welds to increase the strength and rigidity of the chassis. It's a given on a good race car, and we welded up the Targa Miata for that reason.
We're not sure where these diagrams came from originally so we can't give proper credit. They've been floating around for a couple of decades. Sorry, we don't have bigger versions.The red and blue dots tell you where to add more welds. For long seams, it's best to stitch short welds together instead of doing one big seam. Note that you will have to be careful to remove as much seam sealer as possible when preparing for welding, so this is a messy job.
The seam around the door opening and up the windshield frame is easily accessible and can make a big difference. One of the other diagrams (not shown here) indicates reinforcing plates being welded over the frame rails - just like our bolt-in frame rail reinforcement kit.