If you have a 2wd, for a pretty slick solution, but more of a project,
the ingenious PO of my van had added a second tank above the
transmission.  I'm pretty sure it's this one, 6.5 gallons:

https://www.speedwaymotors.com/Black-Poly-Fuel-Tank-6-1-2-Gallon-8-x-30-Inch,5404.html

A second fuel pump fills it from the main tank (~5 min), and a valve
lets it drain back by gravity (~1hr).  I upgraded the valve to a
solenoid, so now it's all controlled by a single toggle switch from
the dash.  No vapors, no leaks, hidden, >400 mile range if you're just
cruisin' on back roads.  Further details upon request.

I was sort of wondering...I believe the syncro fuel tank occupies this
same position above the transmission.  Has anyone fitted a syncro tank
to a 2wd for massive fuel capacity?

Alex