Check the block casting number, which is cast onto a pad, below the rear end of the deck, on the passenger side. All '76 400 blocks are said to be 500557 blocks, which are very undesirable. Hope you have a 481988 block.
Thanks for all the help guys. It's a V8 for sure but something wasn't adding up. I'm getting the other stamps today to confirm the dates. It's just frustrating because I was told it was 67-69. It's to replace my 68 original engine that threw a rod.
"...I'm getting the other stamps today to confirm the dates. It's just frustrating because I was told it was 67-69..."
Hey, nowadays, there are LOTS of guys, including Pontiac guys, who will lie & rip you off, if they possibly can.
The block date code is usually a 4-digit code, located back by the dist hole. The 1st digit is a letter, beginning with A for Jan, B for Feb, C for Mar, etc. The next 2 digits are for the day of the month. The last digit is the last digit of the year the block was cast. 8 = 1968, 9 = 1969, 0 = 1970, 1 = 1971, etc. 7 could be either 1967 or 1977. No Pontiac 400 blocks were cast after 1977.
So, with the date code, casting number, and the ZK engine code, you can narrow your ID down, really close. Would need the partial VIN number, to match up with the last 6 digits of the vehicle VIN number, to tell what vehicle the engine came from the factory in. But you don't need that info for what you need to know.
" 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.