I'll check in the morning to see if this is normal and expected. I don't remember hearing anyone mention it before, but I don't know if that's because the bolt in your kit is a different size, or everyone else already has a thinner washer installed.
I'm pretty sure that, even with my stock setup, when I installed my first set of polyurethane bushings I had to use a thinner washer too. I've had it on there for so many years, I've forgotten the exact details, but I took off my stock fatty eons ago and still have it in a drawer or jar somewhere.
So the bottom line is, no, you don't absolutely need that washer. But I think it would be nice to mention that in the instructions if that's the case.
Along those lines, have you run poly bushings on this rig before? Maybe it's just an occasional thing, maybe it's the norm.
I do think that it would be nice to have the bolt long enough to re-use the original super-thick washer that came with most EB's. But maybe it's another case of "your results may vary" and it's hard to find a bolt that's just a 1/4" longer? Or a thinner washer puts less stress on the bolt's welds? Long shot, but hey, it might be part of the "engineering" of the setup.
Nice fall-back answers anyway! %) but not the bottom line yet.
What do your other Broncos have on them? Thinner washers, or no washers at all?
I'll try to get back to you as soon as I can in the AM. If you don't hear from me pretty soon though, give me a shout at 209-943-0991 and we'll go over the options.
Paul