For those who cannot get hop extract, or dont want to hunt it down, I used Magnum on my clone and I found that to be "sufficient" as a clean bittering hop. Love Magnum actually, use it to bitter 90% of my beers so I might be based.
I do the same. 90+minutes isn't really going to impart a noticeable degree of flavor, and it's definitely a clean bitterness. I only use hop extract for tweaking a recipe post-fermentation. Just because Kimmich uses it doesn't mean it's necessary or even necessarily a good thing.
Which leads to my other point, that people are going about this cloning thing all wrong. I REALLY hate that I'm agreeing with bobbrews (because of the general inanity of his posts), but it doesn't help to blindly restrict one's own ingredients/processes to what the original brewer says they do/use - EVEN IF they're telling the truth. It seems to me that cloning efforts are often HINDERED by the (incomplete) information or hints one gathers. For a homebrewer to end up with as close a clone of any commercial beer as is at all possible on their own system, you can pretty much EXPECT that recipes are not going to be identical. Even if we had the exact recipe, it's all but certain that it could be tweaked to produce an even more faithful clone.
I haven't had Heady (yet... but it's at the very top of my list), so I can't contribute much so far. I will, however, strongly suggest that the better brewers among those with access to it (particularly those who have an extensive ingredient identification/selection knowledge-base to draw from) should BY ALL MEANS deviate from what we're supposed to "know" about the recipe, if that's what their gut is telling them.