Hey Flop (BTW, is that your real name??) I think what you are calling an "undercut" is the term I am struggling with and not understanding.
The general term for a weld that has not been planished is a proud weld. As you well know to eliminate this, the weld is made and then planished using a hammer or slapper and also using a dolly or spoon backup with the dolly-on method. This not only flattens the weld but it strengthens it and it normalizes the molecules where you do not have warpage. What I am trying to figure out is, ...are you saying the weld is warping the sheetmetal on the edges --or is the "undercut" something caused by the grinding?
I'd bet you were ready to strangle your buddy when you needed those tipping dies and he had them locked up!!
They would have made quick work out of it. Have you ever used a crescent wrench to tip with when you need to do a curve? That might have worked well here and then used a shrinker to take care of the excess.
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