I decided to start with the parts and geometry CJ3BL is using on his build since that's what peaked my curiosity.
To start with here's some numbers, mostly just an educated guess on my part, but this is really just for example anyway. (After doing this spreadsheet, CJ3BL told me the springs are actually .24" thick, but close enough for now).
I just used a generic spring rate formula that I found plastered all over the web:
10000000 x Number Leaves x width x thickness^3 / free eye-eye length of spring = spring rate [in/lb].
His initial geometry is as follows (YJ included for reference)
With that I then drew up a case with a 90 degree shackle angle (where the angle is the measurement between a line drawn through the spring eyes and a line drawn through the shackle holes). I had to iteratively play with my model and spread sheet to find a spring hole-shackle hole distance that resulted in a 90 degree angle at the expected 700lb load (based on the above table). I know it's hard to read, but I put the numbers in the sheets below. Also note, this example is just for the rear of his Willys.
If someone was a little better at trig than me, they could surely do all of this in a spreadsheet... anyway, here's my simple spreadsheet (It's just based on numbers from my CAD sketch)
To define some terms here:
A) Spring condition. Compressed flat, ride height and at free arch/no load.
B) Relative shackle angle is the angle between a line drawn through the spring eyes and a line drawing through the shackle holes.
C) Angle from vertical is the measured angle between the shackle and a vertical line
D) Axle vertical travel is the vertical distance the axle has traveled for each condition.
E) Spring arch height is the distance from a line drawn through the spring eyes and the furthest point of the arch centerline of the main leaf of the spring.
F) Distance spring compressed is the relative distance the arch has traveled relative to a line drawn through the spring eyes. (spring arch height minus free arch height).
G) Force in spring is simply the spring rate x F (dist the spring has compressed).
H) Force at axle is factoring in the motion ratio of the spring to actual motion and force felt at the axle. =F/D*G
This last one is the one I'm mostly likely to have messed up, but the way I'm thinking of this is similar to a cantilever shock for example. If you have a 2:1 ratio for instance, you would have double the travel a the wheel or 1/2 the spring rate at the wheel as felt at the shock. In this case the ratio changes throughout the spring travel, so these numbers only apply at this particular point.
I) Is the average spring rate in this position = H/D (force at axle/ total travel to get to that point)
J) Is the horizontal distance from the FWD spring eye to the shackle mount. In all cases the vertical distance is fixed at 6.438"
I then repeated this same thing, adjusting the sketch and solving for "ride height" at shackle angles of 80, 70 and 60 degrees. The only adjustment made is to the ride height shackle angle and the "J" distance.