Rob, turn the 400 over and look at the main saddle areas and the sides of the webbing where they go up to the bottoms of the cylinders and to the oil pan rails. Then look at a 383/440 and you'll see what I mean. It's not a ton of difference but there's more meat there than any other factry block. The "230" blocks are even beefier in those areas.
72 - You may also need to have the shop turn down the OD of the counterweights about .200" so it clears the B wedge main webbing. You'll want to trial fit it before you have the assembly balanced.
