I am not sure if you could make the cut and glued liner leak proof.

Not sure I understand. You say you want a galvanized tank and you also say your indoor tank is leaking. Is the leaking tank a galvanized tank or are you considering replacing your leaking tank with a galvanized one? If your leaking tank is galvanized and you want to put EPDM liner in to prevent the leaking I wouldn't recommend that. It's almost impossible to get a leakproof seam with caulk. On the other hand I have kept fish ( koi, comets and gambusia) in galvanized stock tanks for 5 years with no problems whatsoever. Not saying that's the right thing to do, just that my personal experience is that I had no problems.

Cant you seal your old tank? My main filter for my pond is a 350 gal stock tank and no issues, oh it leaks a tiny bit but I caulked it with aquarium sealant.

Forgot about this thread. I was thinking of replacing the plastic tank with galavanized as I like the looks of them and the fact that they are longer/narrower fits my space better.... but have heard that the zinc in the metal would leach out and not be good for the fish. So the purpose of the liner would be to protect the fish rather than keep it from leaking. However just using one piece would be awful - the folds and pleats would take up half the space inside the tank.

I will have to speak to Koi Jack and Norm Meck about the zinc leaching out and harming the fish, since I am using one myself I will post their replies.
Jack Chapman AKA Koi Jack is AKCA certified Koi Health Advisor and Norm Meck is a well reconized Water Quality expert for more info follow the provided link

Here is a link that might be useful: Leaching