I posed question to a friend who manages one a nationally known engine building and machine shop and businesses. I will not identify the place but you have all heard and know of them. His response is as I thought it would be.
"Dart has the finest blocks available. Not to say that the Merlin III blocks are not a good foundation for a BBC build, they just aren't the same level of quality as the Dart. Both have features which are a vast improvement over OEM such as priority main oiling configuration, additional head bolt locations, superior material, etc. I have been to Dart's facility in Troy, Mi. and also to their machining center where the preliminary procedures are performed to the raw castings from the foundry and can attest to the commitment they have to producing the best high performance components on the market. My personal experience utilizing Dart products in all manner of big and small block Chevrolet and small block Ford applications have always produced positive results with the Dart pieces performing to the expected levels and beyond. "