You can get about 35 gallons or so into a 25" x 57" space, which you should have under there. The trade-off may not be worth the effort for an additional 5 gallons. You would need to cut or drop the bellypan and add some rails to hold the tank up, plus some plumbing would need to be done. If the space in the gaucho is not needed for other storage, I would get a replacement tank and put it in the same place it was. You might even find that you can fit a slightly larger tank in the gaucho.
It makes good sense to add a grey water tank into these older units, and that's a great use for the under-floor space. That's all I plan on adding down there.
Rich the Viking