I have "done " of of the fuel tank jobs on a Avanti.
I had to pull mine, flush it, then put in about 5 gallons of clean
aggregate, and roll it around the yard for a few days. The cleaned it out,
painted it black, and let it sit in the Oklahoma sun for 2 days.(just to dry
it out.) Then installed some of the gas tank liner from JC Whitney.
That was 17 years ago. Still works great.
Jim.

I don't think new tanks are available... I bought one of the very few
remaining tanks about 20 years ago (I think it was for an Avanti II,
slightly different filler neck for unleaded fuels, but I figured it was
easily adaptable) and it was very expensive, about $450 at that time. I
bought it for a car that had a rusting leaky tank; I needed to fly in
and drive the car from a remote location, had only two or three days to
prepare the car, and I wasn't sure the tank was repairable, especially
in that timeframe, so I had the new tank shipped to the car's location.
Actually, I never did install the new tank, I found a radiator shop that
cleaned & repaired the old one in a day, and I kept the new one as a
spare (never got around to selling it... now I live in California where
tanks don't rust, so maybe I'll put it up on eBay one of these days).

George Dimitsas of Olympus Avanti used to advertise tanks in Avanti Mag.

http://www.theavanti.com/Parts.html

Jeff