" No reason yet to believe it is a 557 block based on known codes..."
The 1976 ZK code 400 would be a 500557 block. So, a 557 block is quite possible. Only other ZK code V8's were in '73-'74, as previously posted. Both years would have used the 481988 block. So, I'd think the block has to be either a 500557 or a 481988.
"...If a relatively stock rebuild is a 557 block an issue?..."
It is if one of the main webs cracks at a dowel pin hole, like so many others have. Therefore, I personally would not put much money into a 557 block. Obviously, there have been lots of 557 blocks that didn't crack. Considering the price of a high quality Pontiac rebuild, why take a chance on a block widely known to have failed many times in the past. But, if you have 557 block engine that is running good, not needing any shortblock work, may as well just keep running it, IMO.