I am amazed that some people who own classic cars are concerned that they lack structural integrity.
Of course the structural integrity may be less than a modern Euro-Box, but that is part of what classic car ownership is about and classic enthusiasts have to accept that.
If classics were so dangerous as some seem to make out the insurance premiums would be huge. However, due to the low mileages classics do and the care with which the loving owners drive them they are statistically very safe.
They don't need to be made more structurally stiff, so long as they are rust-free.
Just restore them as necessary and enjoy the retro driving experience of how it was in the 1960's, which is what the classic Mini, and many other cars, represents. Why else would anyone buy one? They are slow, uncomfortable, lack modern reliability, they go rusty, they leak, but they are FUN, FUN, FUN
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I'd rather crash a mini than a smart car.