Corrosion is a significant contribution to spring fracture and will set up stresses and strains, particularly at the ends of the coil where it leaves the pressed housing on the seat. I am not sure how effective painting will be in terms of ongoing protection, since the paint may well crack as the spring flexes. Modern springs are coated with a flexible PVC type material.
I can't tell from the photographs posted whether the car looked to have sagged at all, and you say that the spring lengths unloaded, are as specified, but that may not mean necessarily that after fifty odd years, they will maintain their intended length under load.
I would have thought that having taken the car as far as you now have, that it would be sensible to replace the springs.
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