Your old galvanised tanks were made from sheet on angle iron. So you are going to hacksaw (twice) through something like 20 mm angle, 4 mm thick, to give the nibbler access to the sheet. Or you stick a mask on and bang through it all with an angle grinder.
I'm not sure I would want to use a 9 inch grinder with thin disks, though, but I would be happy with a small one.
A decent jigsaw would do it as well, as would an Evolution circular saw with the "all materials" blade. One of these wouldn't be much more expensive than a hiring a nibbler, unless you actually live next door to a hire shop.
One of my lads was recently cutting up sheeting from a shipping container using a fairly good jigsaw. It wasn't fast (or accurate). I'd told him several times to use the Evolution, but he didn't believe me. I had to demonstrate, eventually.