Leaf springs too soft

I don’t like how soft the rear leaf springs are. They are new leafs I replaced during the restoration and they obviously aren’t the heavy duty ones.
I don’t want to replace them again so am wondering if adding a leaf would do the trick. I know re-arching them would raise the rear end but I don’t think would make them any stiffer. So which leaf would it be best to add? I’m sure adding the shorty #4 would be useless, but how about one of the others (other than #1)? I was even thinking of taking one of the leaves out of my old set and adding it on each side ??? Suggestions or ideas?? Thanks.

photo of spring setup

Here’s an less invasive option; http://www.jcwhitney.com/quick-change-helpers/p2005887.jcwx?filterid=c11305u0j1 have these on my '69. About 1/2 hour to install,…took the spongy compression out of the rear suspension. Use the 7 & 1/2"H version. Top of coil will rest on bump stop plate when installed. Definitely stiffer!

Adding a leaf will defintely stiffen it up. I have an extra leaf on my '68 which was added when i had the rear leaves re-arced in ‘83. It’s’ still stiffer than stock to this day.