I "found out" by complete accident that I "wanted" way way stiffer than I thought I wanted. My 67 of course had whatever the factory 273 were
I had bought a junker that came with ? .9" bars. (unknown aftermarket)
These bars turned out to be "just right." And here I thought I'd been happy with factory 340 bars, etc.
On my rear springs, I had junked out two later cars for parts. I tore down my original rear springs, and one set of the junkers. I combined two sets, putting the longest leafs of the two sets together, with, I think, one more leaf total than what I started. These turned out to be "just right" and it was all completely accidental.