Yeah, that's the problem with that drain plug - you have to replace the seal every time you remove it, or it will leak. I would try getting the engine up to operating temp and then try loosening it. The aluminum block and steel bolt expand at different rates when heated, so it may give you a little free play, making the job much easier. Of course, trying to remove a 180°F bolt that's situated next to a 300-500°F cast iron exhaust manifold and is holding back a couple gallons of 180°F coolant isn't going to be a fun job, either.
I would suggest draining all the coolant and heating the bolt with a torch, but the only way to drain the coolant from the engine block is by either removing the engine and tilting it to drain, or by removing that drain bolt.
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