My 454/455 springs haven't appreciably sagged in the last 20 or so years....That I have noticed!
but, full disclosure, I bought them used/second hand
and the seller claimed a reputable local spring shop, John R. Spring had reworked them.
I think the springs already did whatever they were going to do and were probably fixed.

They had added a leaf, but beyond that.....I don't know what other work they did.
The ride height was ridiculous with the added leaf.
I took the added leaf out, shortened up the center bolt to match, and have used them that way for many years.

My experience is not typical if I can believe what folks have told me over the years... and I usually don't mention it.....
....but if budget is an issue it might be worthwhile to have a spring shop bring them back up to snuff.