Removed Crusty Corroded Coolant Drain Plug -- Now what?

[Benzwood]

I rounded off the long-suffering hex head on my engine block coolant drain plug, so I ordered a replacement. When I got the new one it became evident why the head rounded off... the plug appears to be made of aluminum.

I FINALLY got the old plug off turning it with a vice grips. It had really jammed in there and for a while I had my doubts whether it was going to come out or break off in there.

The plug was so corroded that I had to use very strong pressure almost all the way out. It was complicated by tight quarters, I was only able to get about 30 degrees of movement per clamping with the vice grips, and with hot coolant dripping on my face of course. Took me nearly an hour.

The threads on the old one are all mashed up, with corrosion evident. It left behind some shreds of aluminum in the engine block hole.

I flushed out the hole with a garden hose feeding into engine block from the other side to get the aluminum shreds out. Then I tried to scrape the threads clean using a little Dremel wire wheel by hand, while simultaneously flushing water through to keep any debris from going back into the block (and onto my face instead).

But... when I tried threading the new plug in, it still only went in about half a turn before starting to get hard to turn.

Should I just crank that puppy in anyway, i.e. is it supposed to turn "hard"? The threads on the new plug seem very tall and sharp, so maybe they just get compressed down to fit? Maybe a little anti-seize on the threads would help?

Or should I buy a (big) tap of appropriate size, and try to clean the hole out that way? I'm not sure if that would work since the point of a tap is usually quite tapered, i.e. I may not be able to screw it in far enough to engage the full diameter of the tap into the threads. Maybe I could snap the tapered end off the tap if I had to, but given the size of the tap it would probably be a $20 experiment. And I'm also worried I might screw up the threads given the tight and awkward area I'm working in.

Another thought I had was to get a big metric bolt and thread that it in and out a few times, maybe with some penetrating lube on it to try to break up any crust in there.

Any other thoughts appreciated, the sooner the better... obviously my car isn't going anywhere with a hole in the engine.

Old and new bolts photo below.