What do you suggest to figure out if it needs a new gas tank or just cleaned? Do I have to drop it or just pull the sending unit to inspect and remove old gas?
Pull the sending unit, replace fuel line, drain all gas. Then try to look inside. Not easy to see. It's not going to look pretty and shiny. But look for loose scale. At than point you are a few screws and a couple of bolts from dropping the gas tank. Recently seen a 69 Dart sitting for 20 years cleaned out the loose stuff with a pressure washer. Of course let it set out on a hot day to evaporate all the water inside.
Typically your pick up screen/sock on the sending unit is disengrated. If so, replace it.