Just drain the gas. Just dumping stabilizer in the tank will not treat the fuel in the lines or carb.. it can still gum up. Drain it into a gas can and use it for your lawn mowers/chainsaws etc.

I run my 1942 Ford GPW with no additives and 10% ethanol (regular pump gas) with no issues. It's an L head and still has the original cast iron valve seats.

The general consensus is that the motors are not driven hard enough/enough miles to justify hardened seats OR additives.