Jason,
1) How the heck do you get to that top bolt??? There is no room!
>First WAIT TIL IT COOLS DOWN
Now,while I don't have a Unique, I believe that if it's that close you may have to cut a hole in the footbox to get that bolt in. I know that, that's been recomended to me for my car if I need to get those bolts off.
2) Does the "clanking" at idle make sense with the bolt missing or perhaps indicating something else wrong?
>Don't know this is hard to identify without hearing it, loose header?, cracked header or flange?
3) Is it "safe" to drive without the bolt without damage?
>Probably not a good idea, this can cause a burnt valve, exhaust fumes in the passenger compartment, maybe a long shot but even fire under some conditions. Better to take care of this ASAP before you find out what could happen.
good luck,
Mike "monster"