1968 Mustang gas tank
I am a 1970 guy but I am almost certain your tank is bolted in.
I don't think you have straps under the tank.
If I am right and understanding you....You need to
pull the thin board that covers the spare tire. It is possible that their is
a large wing nut that holds the board in place.
What I suggest is:
1. -Remove all stuff from trunk
2. - Look for wingnut or a cap about 3' WIDE.
Unscrew the cap.
3. - Remove the woodlike panel and remove the trunk carpet.
Then you should see the top of the gas tank.
Now along the side of the tank (looking in from the trunk you should see about 12 bolts that hold the tank in place. So that you understand, the nuts are on a lip on the tank but the heads are in the trunk.
Remove the bolts, then the wires and hoses from under the car.
You will also have to remove the filler neck.
See what you locate? Print Dad