My guess is thats already some sort of a repair panel in your photo; the orignal 'almost certainly' would have included the lower flange and then they were all sandwiched and spotted together. I could be wrong though!
However, I cant see how you could "easily" spot weld that, and i would say that a seam weld would be relatively straight foward without distortion so long as you take it easy. If you spot it you will surely end up with a hard to seal seam right in front of the wheel? Besides, any distortion is likely to be far enough under the curve to be hardly noticeable? Just food for thought!
That's an interesting way of finishing a sill panel....
I have replaced quite a number of sill patch panels over the years, both bought and hand-made, and I cannot remember any of them not meeting at the lower flange. Whether you are seam welding or spot or plug welding, the flange is very much easier and quicker to weld to, providing it is in fair condition, of course.
I must share the opinion that the previous panel with the plug welds in was also a non-original panel. But I have never welded one of these particularly nice cars, so I could certainly be wrong.
But if they fitted sill panels like this as OEM, they must be fairly unique.
I was feeling about mine the other day as the same repair will be needed, probably in the next year. I think the "original"shown may be a previous repair, from the factory I think the rear wing finishes at the bottom vertical flange.
Genuine Mazda rear wing with which includes the sill, you can just make out, no vertical section at the bottom. As far as I know all MX5's are like this, whick is probably why there is such a rust issue.
So I can spot weld, as thats how it was originally, but seam welding would be better for rust prevention?
I don't want to labour the point and there is a strong chance I am wrong, the panels I can find are replacement ones, so I wonder if they're like that so you don't have to deconstruct the 3 layer seam at the bottom. What I don't get is that iftat was how it was done at the factory, how did the spot welder access the top side of the wing to inner sill to get the spot welding electrodes either side of the join?
Some of the rear sill only repair sections do have the seam at the bottom.
If you are worried, why not just seam weld a flange along the bottom (weld on what will be the inside) grind the flange to panel welds smooth then plug to the flange and seam weld at the top. Also plug weld the wheel arch section to the inner wheel arch. use zinc spray on all these overlaps and waxoyl after painting.