Hi jdeuel87,
You might want to start by calling Unique
Reason I say that is the springs depend mightily on the locations of the mounting points, both on the frame (kit builder does this) and the wishbones at the lower end.
Wishbones are commonly used in different widths: XKE which are the narrowest, the 3.8S/420 which are a bit wider, and custom! Some folks shorten the XJ6/XJS units (must be done correctly
or can be disastrous) or even make custom wishbones, e.g. Petrus or Concours West.
Where this all matters in spring selection is in considering the angle that the shocks make with the vertical - all the above elements can change the mounting angle and length of the shocks, which in turn will affect the how the stiffness of the springs is transmitted to the wishbones.
On the Everett-Morrison cars, generally a 300 lbs / inch spring (all four) is about average for their production lately. I'm using a set of 350 lbs / inch springs and am quite happy with them, using a 3.8S set of wishbones.
The ERA web site also probably has a good writeup on this too (have to admit I didn't look, but their manual is quite good
).
In your shoes, I'd start with Unique's recommendation and tailor from there as you see fit!
Tom