Are you using the multiple dot weld technique advised by MPandC here? Weld a series of well spaced dots and planish them to remove the shrinkage, then repeat till the whole butt seam is welded.
I see that you're using the weld clamps that force you to have a space between the edges. You'd be better off without them, clamping the pieces together with minimal to no gap.

Another technique that's been advocated by some is lap your patch over the original metal and then use a cutoff tool at a 45 degree angle for a short distance, weld that section and then cut some more. I haven't tried that but I have welded some pretty lengthy patches in my bed sides and was able to deal with what minor warpage occurred by selective stretching and shrinking.

My first attempt was the bottom section of the front panel. Link
Here's another one that was pretty involved, with two intersecting patches. Link

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.