I took my tank off the other day, make sure you drain it first.
I heated the nuts with a propane torch and they came right off with normal effort from a 14mm wrench.
Be very careful around the plastic cover, the torch will obviously melt that if the flame comes in contact with it, or your overheat the area and get it hot enough to melt the plastic.
ALSO, I'D ONLY DO THIS WITH THE TANK ALL SEALED UP AND YOUR SURE THE TANK HAS NO LEAKS, OPEN FLAME NEAR GAS FUMES, BAD COMBO.
Ummm, you just said your tank is leaking, well there goes that idea. But, so does cutting the bolts with anything that creates sparks.
I'd drain the tank and get air circulating around there to move any of the fumes, if you've got an air compressor, you could pull the fuel pump out and force air down the filler neck and purge the fumes out of the tank.
To drain the tank, since my XJ tank doesn't have a drain, I disconnected the return line from the tank and ran it to a gas jug, then started and ran the motor, all the return fuel pumped into the jug instead of back into the tank, ran it till the tank was dry and the motor stalled out.