When I took my 87 spoiler off, I took the bumper off also.
But these are the 4 bolts that secure the valance to the body. green ones are accessible from inside the trunk.
I am not sure if it's easy to do without having the bumper off. Also check out this link for a guide on how to take the bumper off.
http://forums.pelicanparts.com/porsche-911-technical-forum/545165-88-911-bumper-removal.html