Yes, Dart and World Products (makers of the Merlin block) do cast their own blocks. They are externally the same as the old Mk IV GM big blocks. However, internally, they are much beefier. Pretty much that is the reason that you don't see many builders like myself using a GM block. There is nothing wrong with the GM blocks as long as you don't try to make 1400 hp with one. They are great for up to about 900 hp reliably. The Darts and Merlins weigh approx. 75 lbs more than a Gen VI 502 block. That is 75 lbs more cast iron. If you think about it....GM saves 75 lbs of cast iron for every block they cast. The 502 block weighs about 225 lbs. So, every third block is free material. Since they are only making about 500+/- hp, the block is more than sufficient. Dart and Merlin blocks are specialty items that are a low volume seller. They can get damn near whatever they want for money, so a little more cast iron is not a big deal. They are strictly after strength.