TTI header install questions

So I ended up just biting the bullet and getting a set of TTI headers instead of trying to mess with the stock manifolds. That being said, is it possible to install them with the engine in the car or do you have to lay them in the bay when you drop the engine in? I assume I can't bolt them up before I drop the engine in either with the tight clearances I'm seeing. With any luck I should be getting the last of the parts in this week or early next week and was going to start pulling the old stuff this weekend. I figured if I had time I might be able to drop the new engine in this weekend as well, but if the headers don't show up till next week I didn't want to have to pull it again to get them on if that was necessary.