I now have the gas tank and filler assembly out so I can start building a model and trying the fit for a new tank.
I've posted a labelled photo showing some of the things I discovered with the old tank. In addition to the things noted in the photo, two of the four bolt tabs / ears holding it to the floor sheared off with almost no force when I moved the tank.
I'll be really happy to get a new tank built and installed to replace this.