Leaf Springs - Would you do it yourself?

Agreed, all the leaves were tired, but it was pretty cost-effective at the time. Not time effective for the long run, though.

I would be surprised if any local spring shop has the equipment to re-temper them. Re-arching them is just mechanically yielding them to back to their original shape.

I think your comment about steel is correct if you stipulate that it must be loaded beyond its yield point.
If steel's loading is below the yield point, it will last forever.

Points taken. Only someone with a forge or big oven could re-temper spring steel. I used Monroe's coil-over "Load levelers" on Mathilda before putting the new stuff on her. The old leaves don't look original, FWTW, and certainly are better than the poor stuff I took off Gertrude when I got her. The old Mathilda set likely could be used again in a conservative fashion.

I had an Elder Moparian mentor who liked to temper with LN2. He would immerse engine blocks, springs, heads, guns, all sorts of ferrous products in his vats. Old boy couldn't fix his dicky cardio-pump though. He sold me my first Mopar, the '66 NYer back in 2004. Hell of a great car, but the main seal went about the time pneumonia got me.