Fuel Tank Issues! Help!

Hi guys, here’s my problem. I have a 72 Dart Swinger with a 16 gallon tank. My fuel gauge shows really full for a couple of days, then at 1/4 tank it starts to “run out of gas” I go to fill her up and she takes 6 gallons every time. I’ve got 10 gallons that I can’t get to! Anyone know the fix? My sending unit is good and I just took the filter off if the pickup because it was getting sucked up and cutting off my fuel, lmao! What do I need to do to get to the bottom of the tank?
Thanks everyone, and thanks FABO!

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