Well, it turns out the new section of chassis rail was a good fit first time...
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So I've gone ahead and punched out the holes for the captive nuts and the drainage holes. The one on the left is not as far in as the original as the throat on my large hand punch is not long enough, but as long as there is a drainage hole at the bottom of that arched section before the rear axle it should be fine...
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I have shot-blasted the trailing arm mounting bracket and reshaped it using some steel plate and large hammers. There is something quite therapeutic about whacking great lumps of steel with big hammers...
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