Spring helpers

I have used them, in my youth......... on Daily-Driver Darts, which barely accept 255s. They gained me some attitude and were cheaper than air-shocks. and were good for towing small trailers.
But
I wouldn't run them on a big-tub car, like a Barracuda, cuz you can fit up to a 325 in those, if you really want to. and if you do, you'll have to move the springs; and likely narrow the rearend, so you might as well fix the issue the right way, with either New springs, or re-arched springs, or by adding another mainleaf.
Out in the shed, I got a 40/50 year old collection of various types of retired helpers.