Again welcome to the site gduncan
Remember the car/unibody was painted as a unit with nothing installed except for the doors, rear bumper guard brackets, rear valance (held with all screws installed along the top and left to hang), trunk lid and end caps/quarter panel extensions installed (but spaced away from the body slightly)
So in the trunk - no gas tank, filler hose, wires...... only not welded to the car is the one visible bolt holding the drivers side bumper guard bracket in place. All of the seam sealers and sound deadener was already applied then the trunk was painted with exterior color (as much as the painter wanted to bend over and cover with paint but in general all visible surfaces that could be seen standing out side the car from a normal position.
As for the other thread that was posted ignore the transition discussion - it concerns 67 San Jose cars specifically.
We have a thread that showed some pictures from a unrestored 67 NJ coupe. Will throw a dozen or more pictures into the thread (sorry no trunk pictures of that specific car). Maybe you'll find some additional details to discuss or apply during your restoration
http://www.concoursmustang.com/forum/index.php?topic=5463.msg30687#msg30687Need more - just ask