My experiences with varnished gas is not good. What can happen with just draining the gas down and putting in 5 gallons of fuel in and trying to run the motor, is that the varnished residual gas remaining in the tank can spoil the new 5 gallons of gas. I have had varnished gas actually coat the valve train and cause valves to stick and subsequently bend push rods. The problems usually don't show up until the motor is shut down and allowed to cool over night. The mixture of new gas and old varnish separates when the new fuel either ignites or evaporates leavening old sticky varnish to be distributed on the intake valves. As the contaminated gas dries and cools over night it becomes a sticky glue like substance. When the motor is re started the next day the valves stick, causing the push rods to be bent by the camshaft trying to open the stuck valves. I have had this happen many times.
When I resurrect cars that have been sitting, I always make sure that the tank is good and cleaned prior to starting a motor initially, thus avoiding any varnish to be run through the motor. While the tank is out or disconnected you can utilize an external gas tank hooked up to the fuel pump. The external fuel supply will allow for clean fuel to be pumped in by the cars fuel distribution system. The carb, filter, fuel feed line will need to be checked cleaned or rebuilt prior to the initial start. These extra steps will help prevent valve train issues caused by short cutting the removal of bad gas.