[Novice Goat]

I am trying to restore a 65 gto, (I'm new at this so be gentle) and was taking off the driverside fender and the bottom nut, closest to the door came loose inside the body. Let me explain because I do not know the name of the panel; the body pannel that runs down between the front and door windows, behind the fender and just in front of the door (seems to be connecting the door jam to the fire wall ), there was a nut inside of this panel that the last bottom fender bolt screwed into to secure the fender. As I loosened this bolt, the nut inside began to spin inside of this panel. I ended up cutting most of the bolt off because I can not get to the nut, nor can I seem to take this panel off easily, but do not know what to do when I try to put the fender back on.
HELP!!