I just ordered a 360 lb/inch spring from VBP. I ordered a spring reduced in length by 1/2 inch from each end. I assume they have springs in inventory that are still "blank" on the ends, so they can be cut for reduction and then drilled for the mounting bolt. I've heard that some people order them reduced in length by 1 inch at each end. I looked at the geometry of the spring on my 68, it's been reduced in length by 1/2 inch also, and I think 1 inch would not be a good idea. With a 1/2 inch reduction in spring length, you can see that the spring to trailing attach bolt is not exactly vertical to the ground, since it has to be tilted slightly to match up with the spring hole. I kinda think it wasn't necessary to have the spring shortened. The spring end on the 68 nearest approach to the wheel has it facing the metal of the wheel. No chance of spearing the tire. I think where you really need to shorten the spring is if you are using long spring to trailing arm bolts so that the spring end approaches the tire sidewall....then there's a danger of the sidewall balloooning out on a bump and then being speared by the spring end.
......Just for curiosity, I installed 10 inch bolts to attach the trailing arm to the spring on my 68. It dropped the rear end down really low, but the spring end was dangerously close to the road and the sidewall. I went back to a shorter bolt, I think it was 8 inches....I could easyily check. The stock length bolt is 6 inches I think.