Replacing gas tank in my '65 Dart...

If it were me I would drop the tank and flush it out the best you can. Depending on how rusted the tank is you should be able to flush it out but be prepared to spend some time on it. All the lines need to be cleaned also and any rubber lines in the fuel system need to be replaced. Check to see if the carb has a screen on the supply tube. Most people overlook that.