I've had several with welded or brazed rear seams, no issues for me. I did (re) line the original tank on the 73 I'm working on now. This is the second original 73 tank I've had that didn't have a repaired rear seam. It was pretty rusty behind that first lining job...
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pic from an ebay sale
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a different 73 with original tank
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We all get pretty excited, but a 5% failure rate would be enough to start a recall campaign.
If I put in a liner I'd tip the tank while it dried so the rear seam had a good thick layer of liner and not worry.