Looks like in that middle pic you posted the leftmost one is the guy in question. Musta been leaking a while to erode the block away like that :-( Anyways, I think if you wind in the right length bolt , you'll be fine. From the picture, it appears there are plenty of threads left to hold a proper bolt decently.
As to the JB weld, it may or may not help- regular JB is rated at 400 degrees ish, so it might just fill in that gap okay. I have seen it work it situations where it sure was not recommended. :-)
Just as an aside, before you tightened the head bolts, you did back them off before torquing them I hope?