Follow up - The JB weld lasted through the summer, and then cracked right along the line of the crack in the casting.
Photo attached.
Now looking at refurbishing a head from a 1947 Chev Sedan....
New link here: https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1474082/drilling-out-valve-guides.html
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