Rear Spring Question.

[minendrews68]

While building my '68 Camaro, I ordered new rear springs (5 leaf spring). I ordered them from Classic Industries. My question is, are there different spring rates for these? I'm asking because when I go over something like a speed bump or a rise going into a parking lot the tires will rub the fender causing the metal to pull from the lip a little. Seems like they aren't stiff enough, but I don't know. Any ideas? I need to roll the fender lips, but it draws me up since it's already painted.