On my 78 Fury, as soon as I left the place I bought it from I tried to fill it at the 1st gas station I came to. I put 8-1/2 gallons in it and it acted FULL.
the gauge still showed just over 1/4 (I think I know the solution to THAT part of the problem). Anyways I ran it outta gas
and poured a can (~5 gal) into it and went home then back to town with a stop at the station. I barely squeezed $10 in there (3.something gallons.) Added to what's left of the can-o-gas I put in on the roadside, that don't add up to full. The owners manual says 19-1/2 gallons capacity. What would cause not being able to actually FILL the tank?? 
And no, its not caved in.