72 Duster with a Rusty fuel system....how do I avoid this???

A full tank is always better since there is less room for condensation to form and corrosion to develop. I have used Sta-bil often being in the Navy and going on deployments. Never had an issue when I got back home 6-8 months later.
STA-BIL® Fuel Stabilizer, Storage for Gas-Powered Equipment
Long Term Car Storage for Your Classic Car: What to Consider | Gold Eagle