Given that, I'd probably say it would be easier and safer to lay it flat on the side panel and install your gear. Stand it up, very carefully and while supporting the HSF, when you're ready to cable route and such. I have an older Cooler Master V6GT and it's probably larger than the H7 you're using and the method I described is what I did and had no issues. Just take your time and use caution.
As an alternative, you can install the HSF onto the mobo first and then install the mobo into the case. You'd likely have no choice but to do it with the case upright if you take that path.
Edit: Wow. Didn't even give me a chance to reply. Well, good luck in whichever you choose.