10'4" is
pdq's stated minimum height requirement for wall mounting. So if you have a brick building, you should be ok, if you need to use a standing frame, then I do not believe your ceiling will be high enough.
I do not believe you would have any room to raise it and mount it any higher. And even if you could, having owned a laserwash and an auto with a 7' 6" clearance as well, I can tell you that I guarantee the number of vehicles that want to wash that you will be able to accommodate with those extra 4 inches will be able to be counted on one hand. Unless you extend the bottom of the L you also are going to be missing the bottom 5 to 6 inches of every vehicle as well. This will leave many a rocker panel very dirty.