They are also very nice to have out when rebuilding the engine as you're more likely to get the block cleaned up when hot tanking.
I've been following both of these threads now as I've been curious as to the outcome. I honestly have to agree with paul on this one. I can't see the block flexing and I think there are more problems going on here than what we can see. I use basically the same procedure described above for installing core plugs and have never had one come out. No staking, straps, or screws. 30 pounds of boost, with a HKS gasket and arp head studs with 600+ hp at the crank...if it was block flex or something like that, I'd imagine I would've run into it at this point. I think there is a good chance the coolant system is getting pressurized here for some reason or another.