The springs can be weak without being broken. Are they sagging? The top set of springs are the overloads and do not normally make contact with the brackets unless the truck has a pretty big load on it. It appears the overloads have recently been against the brackets by the discoloration of the rust on the brackets. If they would have been in contact a lot, the surface would have been shinny or not as rusty.
Have you recently put a heavy load (larger then normal) on the truck?
Springs tend to break at the center bolt, or at the edges near the U bolts. I would look pretty carefully.
A 75 is getting pretty old, if you believe those are the original springs, I think I would replace them. It looks like your changing the rear axle, replace the springs as well. Gene