I have on old pickup, dodge-200 1962, that someone has added saddle
tanks to. I have not used these tanks so the gas in them is atleast
15yrs old.

I have used the main tank for all these years so everything from the
main tank through the 3-way switch and to the engine is OK.

What is the best way to clean the tanks up so I can use them?

The tanks are well-build thick gauge galvizined steel, brass fittings
and copper tubing runs to a 3way switch.

More details:
When I removed the gas cap it has sealed itself with varnish and there
was a lot of creamy colored residue inside the filler neck. When I
removed the gas line no gas came out. It too was sealed with varnish. I
had to poke a hole through the varnish with a wire to get the old gas to
run out.

The gas is a deep reddish color.

Any help or advise would be welcome.
Al