Unless you plan to make over 500 horse power you don`t even need a 4 bolt block. a 4 bolt is said to be good up to 700 horsepower. With that being said, unless you just want a bow tie block, which won`t give you anymore power, I would just get a used block and place the money elsewhere such as the heads and valve train where more power can be made.
From what I understand new blocks have a highe nicekel content in them - less fex.
There are older blocks with higher nickel countents. These blocks were stamped with the #'s 10/20 under the timing chain cover . To find this is a pain. For a few hundred bucks I's rather buy new, unless I can find an LT1 block.
Dart little M's are the only block I'm aware of that will take 700 horse reliably. 600 on steel splayed caps would be opinion for rocket blocks.. If were talking street and drag then you might get way for a short time but it's a ticking time bomb. We get around 1500 laps on 1/2 to 3/8 mile tracks before the main web split on GM bowtie blocks. Almost twice more on the dart. We make 750 with horse on alky and turn 8800. There's much loading on drag and street stuff though. You should alway's build over the planned power level.