The old section was removed by cutting with the grinder and drilling through the spot welds (all from the bottom side) - which was easier than I thought it might be. This left a hole in the bodywork above at each spot weld, the idea being that I would plug weld though this on to the new section.
The new bit was fitted carefully, clamped, and tacked at both ends (with careful attention to all the measurements). We then did the welding we could reach at the back end (I'll do the bottom side when I tip the car the other way). Once welded I cleaned it up with the grinder, welded some more (to fill up a couple of small holes!) and then got it nice and smooth with a flap disk and sand paper. I finished off with a quick coat of primer to see what it looked like - not too bad I think!
I've done a few of the plug welds along the edges to hold it all together, and I now have the car upright so I can finish them off easily. Piece of cake!
Technically, our rules specify that major chassis repair has to be done by a certified welder, so I am hoping that this looks good enough that it won't be noticable at a quick glance. The whole underside will be repainted, so there will be no way to tell the difference between old repairs and new ones anyway.
Now, some pics!
The old section removed:
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New section ready to weld:
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Partly welded:
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Back end welded, ground and sanded:
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