Technical Prestretch a 72" long butt weld? Updated...

Make your upper seam close, 1/4"ish, to the 90deg bend for the threshold. That bend will capture the metal to resist warping. When fitting **** the two metals tightest, if you leave a gap then it will shrink. I tack and then run it out but I TIG and it is a different process.
This is a rocker I welded yesterday and have started grinding the 'pride' off, after it gets knocked down I'll use a file. I can access the inside but very little shrinking even though I had a ***** of a time welding due to lead in the existing metal.

 

Again, there's no pics or sketches in this thread.
It matters greatly where the weld is as to what kind of distortion will present.
A typical 60s rocker profile. The further from the bends that the weld is the more distortion.
Move the weld up after the second before the third and distortion will be minimal. Form up the entire piece and plug weld it.
Could do 8 sets in 42 hrs

 

So here is the first side done. Welded/finished it all in one day. 1/16" max distortion.

 

Nice work.
It'll need mud either way. We do 1" tacks. Tack between those. Weld each gap, skipping three gaps each time. Grind, fill. Or just panel bond the rocker on. Simple and no heat!

Pre-stretching is a funny concept. Without knowing many scientific factors you would be guessing how much pre-stretch. If you stretched it too much, what would the plan be to shrink it?