What I did was to cut the old tank with a roto-zip with the rotary saw attachment. A panel ripper bounced around too much and the saber saw found it flexed too much to work. This cost some but was a good investment. In fact, if you decide to go that way, let me know and I will lend you mine.
As far as tank replacement, I elected not to cut off the swim step and re-glass it. There really isn't another area to use, so I got two six gallon (Todd?) tanks and Siamesed them with 1 1/2 hose and the vents also. I also put in a gauge and strapped them down.. There is a risk, Peggy will tell you, that there could be a blockage in that section, but I did not experience it. Maybe two hose connection would be better. You have to use the fitting kit to add the second hose, but it's no big deal. Someone else may have a better suggestion, but this worked for me.