For the thickness of the metal of a sill and the inner sill, I would recommend a 3 phase spot welder. I have replaced thousands of panels with spot welders, and the only time there has been an issue is with a single phase unit. They simply do not have the power to hold structural panels together properly. When you move along the welds and you hear a 'click' as one of the previous welds has let go, it leaves you with no confidence in the entire run of spot welds you have placed.

You would be better off drilling and MIG plug welding. Again, get a decent MIG welder, not a super cheap unit.

I use one for rear beaver and a few other places. No issues. Weld is strong. As suggested above when it comes to sills. I will plug weld between the spot welds. With the plug welds cleaned up it will retain the factory look.