Pat, I have purchased springs from ESPO, and found them to be very helpful. Prices were competitive as well.
However, they didn't have specs for my 66 GS, and tried to sell me standard Skylark springs.
Is there a part number on the springs you got from ESPO?
Since we now have GM specs for the original springs, ESPO should be able to match the originals to the present offerings.
P.S., The 65 GS Service manual (page 80) also shows the spring rates as being 420 lbs/in rear, and 115 lbs/in front.....that confirms the pn and Assembly manual info!