I got inspired by the question so I dug into the front sill on my truck that needed replacing.
I started by trying to use a spot weld cutter. The spot weld cutter was not successful. Partly because there isn't a lot of room to get the drill perfectly perpendicular to the weld. I did have some success using a larger drill bit and drilling out the spot weld. Eventually I gave up on it as well because the bit became dull (I need a drill doctor).
My final solution was to use an electric grinder to grind down 80 to 90% of the weld then used a pneumatic hammer to break the remainder of the weld. My hands are numb and I think at some point I briefly set myself on fire. Hearing protection with the hammer is a must. I have an old lab coat that I put on when I have to protect my arms from sparks. I think all the lab coat did was trap the sparks against my arms
Not every point where the sill and the bed "folds" meet have a weld. In the middle it can be every other, or every third. At either end it is just about everyone for about three to four spots. I did end up tearing the spot weld through the bed in a couple of spots. My bed is heavily pitted and will be lined with Rhino lining so not a big deal. I'll just use those holes to spot weld from the top.
The round hole at the top of the pic is one weld that I tore out, not terrible and can be easily fixed when the new sill goes into place.
This is the sill off the truck, you can see how I ground into the sill to grind down the weld.
The bed, post sill being removed.
Here the sill has been completely removed. You can see the right hand side where it was rotted through. The sill was full of sand and dirt. Even though I have cleaned, and blown this sill out a couple times I had to stop and vacuum up big chunks of rusted metal and more sand. In all this took about 45 minutes even with all the changing of technique.
I won't be installing a new sill for some time, the bed is going to be sandblasted so I'll wait till that's complete before the new one goes in.