What you have in the way of a failure is not uncommon, and although 'failed is failed', and from here there are no promises, but I've known of quite the number of cast iron blocks with cracks running from the threaded bolt hole to the water jack and they were put back into service without a problem (the engine generally failing from some other issue first). Of engines relevant here, not just FE's, but also the 351C's this is quite common, flat-heads in the center two bolt holes in the decks are more often than not cracked as such. and with the inclusion of ARP studs (replacing standard bolts) with their greater pull forces opens up examples in applications generally not so commonly experienced.
Obviously, the concern is that as especially with cast iron the crack will migrate until a greater failure will be encountered, and that is very possible, perhaps even very likely given enough time and agitation, but it seems that generally once away from the spreading force created by the loaded thread in the bolt hole bossing the creep-rate slows considerably. It seems that most whom encounter this issue will often choose to utilize a stud (vs. a bolt) with the meanest thread locking compound they a familiar with (something that might also wick into the crack might be preferred if only to aid in preventing coolant leakage); but just remember, you might need to remove this fixture at some point in the future so keep that in mind in your choice of glues.
After all, otherwise are you just going to toss the block? :'(
Scott.
