1. Thermostats are stainless steel and brass. They don't corrode. Wild Guess: The prior owner gutted a thermostat deliberately to "solve" an overheating problem.
2. The engine block has a drain plug hole on each side. One side will likely have an iron/steel plug, the other side might have a knock sensor screwed into the hole. Remove
BOTH to drain the contaminated coolant, and later the contaminated flush-water, from the block. Sometimes the drain holes are covered with a "crust" of sediment, you may have to poke a screwdriver into the hole to break the crust. On MY vehicles, I scrap the iron plug in favor of a brass plug, or a brass draincock. The knock sensor goes in with sealant on the threads, and it's torqued into place using the factory spec for tightness, or it doesn't work properly.
3. Might as well order a new radiator. You may need a new heater core. Consider new upper and lower rad hoses, and new heater hoses--clearly the prior owner wasn't interested in maintenance. You can try to salvage what you have, of course. Back-flush a few times, fill with clean water, drive it for a few days, drain it all again. Draining the block directly makes a big difference in how fast the system gets "cleaned-up". If you're only draining the radiator, you'll be flushing and draining for two weeks. And there's various chemicals and compressed-air-and-water flushing tools. "Good luck". I figure the previous owner gutted the thermostat because the radiator was already in trouble.
www.thexton.com/heater-core-back-flush-tool-with-assist-p-115.html
Thexton's Heater Core Back Flush Tool with Air Assist fit 1/2
www.thexton.com
4. There's "Flush and Fill Kits" available that work pretty well--but you STILL NEED TO DRAIN THE BLOCK at least once, and probably more than that. Note that I don't like the included clamps--you need clamps that don't have a zillion holes in them that the rubber extrudes through. Spring clamps, or shielded clamps work better.
www.amazon.com/Prestone-AF-KIT-Flush-Fill-Kit/dp/B000CCFY5W/ref=sr_1_21?dchild=1&keywords=Flush+and+fill&qid=1634419264&sr=8-21
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5. A radiator fill funnel like this makes the job way more pleasant. Less mess, too--because the radiator can burp air into the funnel without making a coolant mess on the ground.
www.amazon.com/Lisle-24680-Spill-Free-Funnel/dp/B00A6AS6LY/ref=sr_1_5?dchild=1&keywords=Lisle+no-spill+radiator+funnel&qid=1634419571&sr=8-5
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6. If I'm right about the previous owner, and the overheating, you'll do well to verify the fan and fan clutch, radiator shroud and air ducting, the seal at the rear of the hood, ignition timing/advance, brake drag--ANYTHING that could contribute to overheating.
7. I flush with tap water--of course. And then drain the radiator AND the block on any engine that has block drains (some Buicks don't. Idiots.) But when it comes to mixing anti-freeze and water, I always use distilled. I have no idea how the various chemicals and additives in "antifreeze" will interact with flouride, chlorine, and whatever else they're poisoning us with in "tap water".
. Went to change thermostat and found what was left (just the little top "hat" looking part). I guess the rest of the thermostat dissolved? Radiator is pretty orange inside (assuming it won't cool well and is destroyed?).
