It may be in the fuel tank but it may not be Firestone'* fault, but that our our government and it'* nasty ethanol fuel. See photo links at bottom of post. The pickups on the full size cars are unique and I was unable to find a new replacement for it, thus a full rebuild is the only option. There is a rubber (for lack of a better word) spacer attached to the bottom of our fuel pickup/fuel gauge unit that can be damaged by the ethanol. This spacer keeps the bottom of the tank from being sucked against the pickup line and shutting off the flow of fuel. The sock on mine was also damaged it'* a unique size and shape and I finally found a replacement from a Corvette restoration catalog (I don't remember the name right now but I'll post it later if I can find their catalog) As for the rubber spacer: I bought a 6" long piece of 4" diameter butyl rubber and carved a replacement, Also these fuel tanks have a non vented cap and a separate vent tube, check that it'* not blocked as mine was. This will cause a negative pressure situation can lead to fuel starvation and add to the likelihood of the tank being sucked against the bottom of the fuel pickup. Once everything was apart I discovered that the float was getting stuck in the pickup (some sort of buildup probably do to the ethanol), take some 2000 or 3000 grit sand paper and carefully polish the plastic float to keep it from getting stuck. I hope this helps a little.
Damaged spacer and sock
Damaged spacer detail