The OP's part number is for soft (170# springs) the number you provided is for firm (300# springs). I can't help as to which is a better setup, but I was curious myself and thought I'd pass the info along.
Paul,
It would really depend what you were trying to accomplish with the car.
Being as coilovers for the G-Body that go in the stock shock locations are going to inherently give your suspension a tendency to have a stiffer ride, we need to keep that in mind when judging the rear spring rate. Because the G-body coilovers that use the stock location require you to run a shorter spring than the stock spring height, which is due to coiliovers having the spring mounted
on the shock body, and because the shock is threaded all up and down the body of the shock, this is where the spring mounts go, which is what our coilover springs sit on. Since the mount for the coilover spring is literally somewhere in the middle-ish/lower-middle, of the shock body, the spring size has to now be
much shorter than the stock spring, because otherwise if it was the same spring rate and height as stock it would push the body up so high you wouldnt be able to mount the coilovers and you'd never get the body down.
So since the coilovers use these shorter springs, they
have to have firmer spring rates in order to prevent the body from bottoming out on the frame. The springs' job is basically to hold up the body of the car (at least for the purposes of this conversation) so if you took away length from the spring, since it mounts on the coilover/shock body, you have to compensate for having a shorter spring somehow, and that is by adding spring rate to the spring (making it stiffer).
So when you look at the coilover kits and the "soft" springs are 170lbs/in and the "firm" springs are 300lbs/in, you need to take into account that the springs are MUCH smaller than stock rear springs, and that extra stiffness is there to...well...basically to stop the body from hitting the frame...but now that we have these stiff springs, we run into issues trying to launch the car at the dragstrip..but I won't go into that here...
I literally
just put a rear coilover conversion kit on my buddies '68 Chevelle and I chose springs with a rate that wasn't drastically stiffer than stock.
First you need to decide if you are going to primarily take the car to the strip, or if it is going to be a street car. Once you figure that out, do you care more about the car being able to take some windy roads like an absolute boss, or do want the car to be ever-ready to hook up and make a hit on the street? Remember, you can always buy springs from JEGS and return them, which I would STRONGLY recommend doing...so don't feel like whatever spring rate you choose is IT FOREVER...because it isn't.
Anyway, sorry for the longass rant/writeup...but I hope that kinda helps a little?
-Tyler
