Carburetor spiback when turning engine off?

Usually, this is an indicator that the carb needs to be rebuilt, or at least, thoroughly cleaned. When the engine is shut off, the fuel supply is supposed to shut off with it. If a carb is dirty or out of adjustment, the spillage from a leaky float can cause an after run, or burp, caused by the unwanted fuel. Sometimes, you can just lower the idle speed a little to offset this. A car of this age is worth having the carb replaced or professionally re-built. Or, make it a project. I prefer Borg Warner rebuild kits. You should also consider having your heads worked on to account for the unleaded fuel that is in use today. This engine was made to use leaded fuel.There is a kit available to adapt it. Or, you can exchange the heads at the Ford dealer. For best results, install a 4 barrel intake with a Holley carb. The front barrels are smaller than the 2 barrels, will improve gas mileage and power at all engine speeds. Good Luck!! Also,convert the engine to electronic ignition. Some of these older cars still have points and condensers, which can also be causing your symptom. Good Luck!!