The spring rate is how many pounds of force it takes to compress the spring one inch.
The "right" way to calculate the necessary spring rate is to measure:
1. Spring length when the lower control arm is level
2. Unsprung weight of front end
3. Distance from lower control arm bushings to center of spring mount
4. Distance from lower control arm bushings to center of tire
With me so far?
#2 is how much weight the suspension needs to lift. If you have a way to measure that, use that number. Otherwise we can estimate. The 1948 GM Heritage Center vehicle information kit says the front curb weight of a 3100 pickup is 1835lbs. Technicallly your wheels/tires should be subtracted from that and the IFS added. For now, lets just use that number to get a starting point. If this number is a bit high, we will calculate a heavier spring than needed and make it ride too high (fixable by trimming the spring a bit).
Divide that by 2 to get ~917lbs. Hold onto that number.
Now divide number 4 by number 3 to get the mechanical advantage that the control arm has relatove to the spring. I don't know the actual numbers, so I will make something up to finish the math. Lets say that 4 is 12" and 3 is 4". That gives an advantage of 3x (12/4).
Multiply that by your weight (3*917lbs=2751lbs) to get how much force the spring needs to have to hold the vehicle.
Now you can look at springs online to get the fully extended length (or measure what you have since I believe they are standard) and compare it to measurement #1. Multiply the difference by the spring rate of the spring to see how much force it will excert installed.
Look for a spring that matches the force (2751lbs) above. More force -> higher ride, so I would be inclined to go a bit high so you can trim to get exactly the height you want.
Clear as mud?
If you post up the 4 measurements above, I can walk through some real numbers with you.
