If your tank has baffles in it, looking in the sending unit hold may not tell you much. I'm in agreement that it's probably old gas that's turned into varnish. Normally I'd recommend cleaning it with acid & recoating the inside but if there's that much in the tank and if new tanks are available for your truck, buying a new tank is probably the best way to go. I didn't have an option with my 38, new tanks aren't available. If you're determined to keep the original tank, there is an outfit that splits the tank, cleans & repairs it, welds it back together then coats it.
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