Horse Sh%t all over the page.
Never has any documentation ever been produced to inclue any blocks having more nickle in the casting process. To add to that, several have tested the theory with alloy testing that showed it's all nodular iron.
To add, Stock blocks with four bolts straight in, tend to be weaker because the bolts pull the main webs differently than those with 2 bolts. With 2 bolts, you can drill them at at angle and avoid this. Adding a cast iron 4 bolts caps to a 2 bolt block helps some because the caps tend to be thicker and the registers have more contact area however, it's still at cast cap.
Lastly, buying an aftermarket Dart block isn't a bad idea for $1750 but they aren't ready to go either. They still need squared up and that basic machining adds another $500ish for a total of $2300. They are a good block that last a long time for sure however a stock block with splayed caps and squared up is still around $1000 typically. So a difference is $1300 buys some longevity and strength and that's a lot of dough for the power of most street machines.
In the engine world, the progressiveness of parts and machining is never ending. It's always a few more bucks for the next best thing. Where is the end point lie? How many times can a guy pay "just a few more bucks" before it all adds up to nonsense?