Having just done this (several times), I thought I would jump in. There are of course several variations of how to do this depending on how you want it to go, so I will provide some information;
Option #1
1. remove the 4 screws that hold the filler neck to the quarter panel
2. remove the rubber seal in the trunk that is around the filler tube and slide it up the tube
3. spray the rubber seal on the tank/filler tube with a penetrating oil or WD40 (let it soak)
4. remove the real valance
5. spray the nuts on the J bolt nuts with penetrating oil
6. disconnect the fuel and/or vent lines
7. remove the sending unit wire
8. brace the tank (especially if there is fuel in it)
9. remove the nuts on the J nuts
10. drop the tank along with the filler tube; it will not come all the way out due to the head of the filler tube
11. if on the ground, brace the take with your knees and using both hand twist the filler tube back and forth and work it out of the tank.
** THE DOWN SIDE OF THIS TECHNIQUE IS AS NOTED IN POINT 11; BASICALLY YOU HAVE TO MAN HANDLE THE TANK AND TUBE TOGETHER. ALSO, WHEN PULLING THE TANK DOWN WITH THE TANK, IT WILL COME OUT AT AN ANGLE WHICH CAN CAUSE ISSUES WITH THE TANK HITTING THE EXHAUST.
Option #2
1. remove the trunk seal and spray the tank/filler tube seal with penetrating oil
2. reinstall the trunk seal (this is for support)
3. remove the rear valance
4. spray the J bolt nuts
5. remove the lines and wire
6. remove the J bolt nuts
7. tilt the tank down and using it as leverage, twist it back and forth to break the seal on the filler tube
8. slide the tank off the filler tube
** DOWN SIDE TO THIS IS IF YOUR GET TO AGGRESSIVE ON TWISTING AND MOVING THE TANK, IT IS TRANSFERRING ALL OF THIS TO YOUR QUARTER PANEL; SO IF THE TANK DOSENT SLIDE OFF THE TUBE FAIRLY EASILY, YOU MIGHT WANT TO GO AHEAD AND REMOVE THE SCREWS HOLDING THE FILLER TUBE TO THE QUARTER PANEL.
It isn't hard, just hard work to a degree depending on how stubborn everything is.