Quick update: Removed and blocked the opening for the petcock. Dumped a handful of nuts/bolts/washers and tossed them around to break loose the big chunks.
Filled with Works (2 bottles) and water. Not happy, I drained off about half the mix. Been slowly repositioning the tank and letting it soak. So far, it's turning out good. I will need to drain it and give it another 50/50 mix of Works to get the rest of the rust out. Very impressed with the results I am getting so far, considering the rusty mess I had.
Next is to order a new petcock. Rubber was stiff, and the screen has a hole in it. And fuel fliters too.