We suggest that you start by draining the old gas as thoroughly as possible, because there’s no good way to revitalize or “freshen up” the old fuel.    Assuming you’re not going to remove the tank, that will make cleaning the inside more challenging.  Normally, we would tell you to add straight B-12 Chemtool (usually by the gallon, which is part #0101) and move the tank around so that the liquid is contacting all the affected surfaces.  However, with tank still in the car, rocking it back and forth is probably about the best you can do.  Because it’s been cooling off, you probably need to soak the tank for a few hours, but you can soak it overnight, if desired.  Then, drain the B-12 Chemtool/gum solution and repeat until the mixture comes out clear.   Once you’re satisfied with the condition of the tank, you should use a fuel additive to help clean out the carb and intake valves.  B-12 Chemtool at 2-4 oz/gal will help, but a detergent-based product would be even better.  Amazon offers Berryman B-60 High-Mileage Fuel System Rejuvenator (part #7516), which is our best detergent-based gasoline additive and at 1-2 oz/gal will thoroughly clean the carb(s), intake valves, and even combustion chambers.   Hope this helps.  Let us know if you have any additional questions.