When are they too far gone?

Wouldn't worry about it. Any spring shop worthy of the name is just going to use the old ones for a pattern. That's how it works at WV Spring and Radiator anyway. The clamps on the front segment is pretty much normal if you want any traction on launch. The spring on the left in the picture looks like it has had some work done to it already and yes, 5 leaves were stock with V8s. Local spring shops are much more reasonable than most aftermarket outlets and if there is a problem they are just down the road. Oddly enough, look for one that does a lot of work on big trucks. My $.02.