I removed a 76 gallon tank out of my old 88 Ambassador 255. Let me tell you, it was not easy. I suspected a leak so I drained all the gas out of it I could and then realized I could just remove an access panel in the aft cabin and lift it out....at least that was the way it looked. I had to cut a riser out of a step and about 4" of the firewall between the cabin wall and the engine. After all was said and done it took me 2 hours to get the tank out of the cabin and another 2 1/2 hours to get it back in. I lucked out and was able to weld it so it cost me $20 in aluminium brazing rods and my time.
If you are leaking make sure you get all the gas out. When I initially lifted the tank the hole in it opened up wider and dumped about 3 gallons of raw fuel into the bilge. I scrambled to disconnect the batteries before a spark blew me up. I THOUGHT a few gallons left swashing around in the tank could be dealt with on the dock...my mistake could have killed me so be careful and get ALL THE FUEL out before you do anything.