seems like those last few percent's of work needed in getting to 100% completion can take a lot longer than anticipated. I had to wheel the cab back to the welding room twice today, the first to deal with those three niggling areas, and then the second time to deal with a little boo-boo when I was defining a sheet metal seam line.
I had a problem in the joint on the dash immediately to the right of the tachometer cut out. I had cracked this once before and welded it, only to see it crack again. I thought I had welded it decently the second time, but obviously not.
So I realized something about welding approach, reasoning out why the weld had cracked: the butt joint was too tight. If the sheets butt tightly together, then what tends to happen is that the weld piles up on the surface of the seam line, slightly melting the upper surface of the seam but not achieving full penetration to the backside of the material. There's needs to be a space allowed for the wire to go, in-between the pieces (just like the gap given by one of those inter-grip clamps

).
Otherwise, when grinding the weld down, the weld that had piled atop of the seam line is 99% ground off, leaving a very thin amount of weld holding it together. A few flexes back and forth and the more brittle remains of the weld give easily along the seam.
A further complication in the case of this crack in the dash, is that the seam was under compressive pressure - the seam line either bulged out (the normal position), or could be made to -'klung!- bow inwards. Neither position was flat to the rest of the dash. The worse of the two options, however, was the normal position bulged out, a trickier and more plastic-intensive situation to dress out.
In the end, this approach worked perfectly, and the dash is now re-welded at full penetration and is much flatter, a tad concave even. That was a relief, and it proved the point to me about leaving space for the welding rod to go when fitting pieces together.
So, I ran the zip wheel through it a ways, and then clamped the copper 0.25" plate behind it and wrestler the dash metal a bit.