I hand pumped what I could out of the tank into an old gas can before dropping the tank just to reduce its weight. Once I got the tank out, there was still some liquid sloshing around so that was poured into the gas can too using a funnel. I left the gas can open outside for a couple of days when it wasn't raining to let it evaporate as this seemed to be the safest way to get rid of it. Just like with those YouTube videos of what happens when people make the mistake of filling their lawnmower gas can with the can insulated from ground (like sitting on a bed liner) and having it ignite, you still have to be careful about static electricity when transferring the gas, even if it's old.
You're probably already considering it, but just in case: plan on replacing anything rubber around the tank including the grommets for the fill line and the vent tubes. My metal vent lines were all corroded beyond recovery as well. Good luck!