I personally try to install rivets so that whatever is exposed, wherever it is, the head is showing versus the crushed side. In this case, the inside of the footbox will be covered with insulation (you're doing that, right?) and carpet, and that will cover the "ugly" side of the rivet. As long as your rivets aren't overly long and crush down reasonably, I'd install them with the heads showing into the engine compartment. BTW, while nothing wrong with it, you're installing the rivets really close. Must like to squeeze those things, huh?