I have a friend who found a similar crack in his 427 after performing all the machining on the block. The shop he used nearly 20 years ago suggested that they bore the
oil hole from the main to the cam over-sized and install a tube to prevent
oil excaping from the crack. This is what they did and the engine was built as a stroker (428 CJ crank) and installed in a '70 GT 500. I am not recommending this but I know first hand it worked with no apparent problems. Oh, and when I drove the GT 500, it begged to be flogged, which I happily did.
The second block, a 428, I know of with the same type of crack was welded with a high nickel electode.
If you decide that you don't have the guts to make this repair, I might be interested in purchasing the block from you as a spare.
Good Luck,
Dan