Resurrection update: removed bad gas - what a smelly, tarry mess! Shoulda just removed the drain plug, but unwisely chose first to siphon gas out. Ruined my siphon, but got it all out. Upper aspect of tank slight rust, bottom tarry areas. I probably should have removed the tank and used a piece of chain for a good shake, then coated the inside. May still do that at some point, but after I put 5 gallons of fresh gas, it started right up - on a 17 year old Optima (before they moved their plant south o' the border). I've kept it on a battery tender and it reads only 395 CCA out of the original 800 CCA. Will get an Odyssey 34/78 if it gets much worse. Knock on wood, the Sears Platinum (rebadged Odyssey) in the Taco is 12 years old.
I'm stocking up on gas filters, as I expect to go through them for a while. I'm hoping the new gas will dissolve the bad gas residue. We'll see.