LS1 Coupe,
I am already spraying 100 shot on the stock block. I am not convinced the block is going to live if I push it much more. Maybe I'm wrong.
your block is stronger than a 400 block all day long. it would be safe to upgrade the pistons, some stock pistons have lasted for years some not so long.
without knowing more about the heads, valve train, carb, ignition etc its hard to tell value. but i wouldnt pay more than 2500 or so.