You can figure out what spring rate you need based on what the coilover is doing now with the weight of the truck on it.
For example, say without preload, your shock is 70% compressed with the truck sitting on the tires. It's a 6.25" travel shock, so 70% of that is 4.375".
So since you're running 600lb springs, you multiply 600lbs by 4.375", gives a result of 2625 lbs of force being supported by the spring.
Now that you have that, say you want your shock to only be 60% compressed with the weight of the truck on it. 60% of the 6.25" travel shock is 3.75".
Since you know the shock sees 2625 lbs of force when the truck is sitting on the ground, divide 2625lbs by 3.75" = 700lb spring rate.
Or, if you were going to use a 750lb spring, divide 2625lbs by 750lbs = 3.5" available downtravel or 56%.
That's all with no preload of course. Not 100% sure if it's better to get the spring rate you need to get the height you want with little no preload, or get a softer spring than you need and preload it some. From what I have been reading you can do 0-2" of preload without issue, provided you don't compress the coil so much that it binds before the shock can compress fully. I guess you could look at the spring rate as a way to get in the ballpark on ride height and then just add preload as needed to fine tune it to get it perfect.