Was looking at the Aerotanks site a couple of days ago. They list a replacement tank that replaces the factory tank and holds 40 gallons. https://aerotanks.com/product/1974-1979 ... ment-tank/
According to the page, you can keep your factory spare location. Presumably you could have this tank plus
a behind-the-ale tank, giving you another 20-ish gallons capacity.
Prior to 1980, the skid plate for these optional. My '77 J10 came with (and still has) no skid plate. Certainly the bare steel tank would not resist rock puncture very well. Perhaps this is where the claims above come from, though I have not personally heard such claims. The factory skid plate should be fine, both for the steel tanks and the later plastic tanks (if not rusted away).
My J20 came to me with an aux tank behind the axle. It has a separate fill, that was not complicated to install. Using the same fill for both tanks seems a bit sketchy to me - not a problem to install a second filler neck and cap, AFAIK.
The electric valve/switch is available in a 6-way version that switches both the feed and return. It also handles switching the gauge between the two tanks. in '77 a 49-states V8 may have vapor recovery - I'd have to look at the book. A CA car definitely will. My '77 258 does not. Not valve needed for that - just tee in the vapor line from the new tank to the vapor line from the factory tank going forward.