That hard chunk is odd indeed. If it is something like epoxy, it would take a lot of it . Perhaps it is that fuel tank sealer that never got distributed properly. I'd be wary of running your engine from fuel from that tank. Pieces from the chunk may slowly dissolve or react with your fuel, and harm your injectors, injection pump, or engine. On my gasoline Ferguson TO-35, I gave up on getting my original gas tank clean and in good shape. I bought a steel 3 gallon gas tank that was used with a small Mercury outboard motor. I soldered a hose barb fitting onto the tank, and mounted the tank on top of the original tractor tank, along with an inline gas filter. This outboard tank has a screw air vent built into the cap. I may or may not ever get around to trying to restore the original tank, but for the meanwhile the outboard tank works fine.