Of all the aluminum v-8s, the only ones I've experienced thread problems in the block were the 380's. Out of several I've never had any threads pull on a 560 block. The 380's that have given problems all had been apart previously. So if it's the first time your engine has been apart and it will be the first head torquing since the factory did it you've got virtually nothing to worry about. Just make sure the bolts and bolt holes are clean and lighly lubricated and you'll be fine.