You might want to lay a grille on your back deck. I don't even think it fits between the seat and rear window. All of the g-bodies I have done go in from the bottom, or we make a new rear deck and use different speakers(i.e. 6 1/2 seperates).
I've made custom plates to hold a 5 1/4 and tweeter and covered with grill cloth. The biggest issue with this is you have to cut a hold through the rear deck cover. If you ever remove the speakers . The g-body systems I do now pretty much do not have any rear speakers, except subs .
John