I have a '71 400 "B" with the BCR mains and girdle.
This is for a 500" stroker build (4.15" stroke, 6.76" rod.) From what I've been told this is a really good compromise of good cubes, light weight rotating assembly, and good rod ratio. It also fits in the block easely. the bottom of the bores did not even need to be notched, just a small bit of the oil pickup boss needed trimmed to fit the rotation assembly. I think the real area of concern with the stock blocks is the stock oiling system. The lifter bores intersecting the main oil galley, and the #4 main/cam feeding oil to the rocker arms are the big issues if the engine will be turning high RPM. The stock block does not have alot of material to support lifter bushings, not the mention the bushings will reduce the size of the main oil feed galley.
I think if you are making less than 700 HP the stock block is a decent choice, and more than 800 HP I believe your better using the aftermarket block. It seems that between 700 and 800 HP it may depend on how much you want to spend modifying the stock block compared to just getting the new block. Also, the new blocks can be built with the 4.5"+ bore size and have room for larger stroker cranks.