"...Is the 500557 block really that bad. I see several people that claim to be at 500hp with them..."
"...This car is going to be driven to ice cream shops, dinner and car shows. not raced..."
For the type of driving you say you're going to do, you may get by. I've already mentioned that no matter what you post, about a problem that some have had, some will say they did it without any problems.
The 6x-8 heads will probably produce less that 7.6 CR, with those pistons, unless the heads have been shaved some and/or the block decks have been machined, to bring the piston tops closer to the deck. But, from what you've posted, all you want is just an engine to get you from point A to point B & back. So, low CR may be a way to help the 557 block live longer. Another way might be to rig up the carb so that the secondaries will not work. I did this on a 350 sbc engine in a 2-ton truck. Ran just fine & got better fuel mileage than our trucks that had a 2-barrel carb, because the Q-jet primaries are smaller than the holes in the Roch 2-barrel.
Pontiac had so many problems with the 557 blocks cracking at the main cap dowel pins, they tried hollow roll pins for the dowels, for some of the 1977 W72 400's. These pins were supposed to cushion some of the shock load, to help prevent web cracking. For the '78 & '79 W72 engines, they cast some more 481988 blocks, adding an XX before the casting number.
Here are some pics of a cracked web, a hollow pin, a block with one broken web, and a block with 2 broken webs. The guy that had the 2 broken webs said that the engine was in a boat & they were just gonna freshen it up. Didn't know there was a problem, 'til they tore it down.
