Disclaimer.
The following is what you read on the internet.
Anyone can say anything.
I had flat springs on a Challenger and bought a brand new set of MOPAR Perfomance springs.
No change. Still flat.
Then I'm sure that I read in the MOPAR chassis manual by Larry Shepherd that Chrysler designed the stock springs to be basically flat for better handling. The only cars that came with arched springs were the T/As and AARs for the side exhaust to clear.
What you don't want is to have the springs arched upwards under weight.
You know, past "flat".
Now, I have added a spring to an A body that is my daily driver.
I did that because I'm using an M body rear in it with 15 inch tires.
That's where I got the extra springs to add, that doner M body.
It's a fold down seat car and when I load it up and hit a bump the tires would scrape.
I added a leaf. It has very little difference in "arch" under load.
I just wanted a tad anyway.
But it is now stiffer and doesn't hit the tires when I go over speed bumps anymore.
Also, if I'm thinking correctly, be advised when you stiffen the rear up in relation to the front you increase overseer.
Conversely when you stiffen the front up by adding things like an anti-sway bar you increase under steer.
Either that or I got it backwards.