First bronco was a stock rear tank only. That sucked
Second bronco it was mandatory to get a dual tank vehicle. Got it, used it.
Put the 23 gallon in the back. Turned into a chore to run the gas out of the aux tank before it turned stale. Was getting close to deleting it when I sold it.
As for installing an aftermarket aux tank. We can guess it is one of the old Aero tanks? Or maybe something fabbed up by a local gas tank shop, lots of those were made back in the day. We don't know. Most of the ones I have seen were built as a one off by a local shop. I really don't think I have seen an Aero aux tank in person. Really doesn't matter. There is no off 'just do this' to make it work. You need to pick up some metal stock, some all thread, and pull out that creativity they taught you back in arts and crafts class. Apply a little common sense. And you will have built your own gas tank installation kit.