As Mick and others have already said, relying on the body / chassis for a good earth is not always reliable. On an older vehicle, even if you clean the area you are attaching the cable to what is the path like the rest of the way?
Even with a new rebuild I wouldn't reply on it. In an attempt to eliminate galvanic corrosion we prime & paint panels, we paint / underseal our chassis, we fit insulating gaskets / sealant / nylon washers and grease fixings. Having done that we then hope that we have failed in our attempt to keep dissimilar metals apart and that we actually have a complete earth path all the way from one end to the other!
When I fitted a new chassis to my 90 earlier this year I did what I have always done before and done away with all body / chassis earths and instead everything connects to one of several common earth points which are in turn connected (using suitably sized cable of course) back to the main earth point (i.e. the -VE terminal on the battery). Simple and reliable.
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