Grade 8 fasteners are the way to go - in the right application. Kube is correct in his worry here, as the threaded rod will not just be seeing a tensile load, but side loading (bending) as well. The metallurgy of this alloy with the heat treat to attain Grade 8 specification is indeed brittle when subjected to bending loads. I cannot overstress the amount of energy involved in this system when you have spread one of these springs out in order to remove or install. Fasteners are not designed for this application! I have an old "official" spreader that I inherited from my grandfather and, although it does utilize fine threads for the spreading, the threads are formed on very large (I'd guess 1-1/4" diameter without going out to measure) pieces of mild steel with forged ends that are curved to nestle in the back of the eye.
Just as Kube said, just because you haven't had one of these fly apart on you, doesn't mean it's not about to happen. And, remember, "watching for bending" and backing off before things get hairy may not be such a good safety plan, as these failures happen fast and you could end up with a face full of shrapnel before you know what happened. You will NOT be able to get out of the way fast enough. Trust me.
I'm not attempting to come across as any kind of know-it-all here, just spread some industry knowledge I have on machine design, metallurgy and working safely - all of which are required in my business on a daily basis.