Get it real hot and use a sealer in the cooling system, it'll seal it up. There's several, I have used Barrs leak quite a few times with success every time.
For a crack or a bad leak, K & R Block seal will work...forever. I have used both and K & R is forever and the best there is.

So will Barrs leak, last forever, on like your new freeze plugs.

I have always used reg clear silicone II right of the caulking gun tube that I got at the hardware store on core plugs on cast iron blocks and have never had a leak. It has never failed me, better than say blue silicone, imo.

several times I have installed fresh rebuilt motors for friends/customers that I didn't build or work on to find a core plug leaking on fire up. I used Barrs leak and it stops it easy.