Apparently GM Removed some material from the casting to compensate for the additional weight in some of these 4 bolt 400's. The design of the 400 is just not suited to performance applications. The mains and lifter valley's have a tendancy to crack because of the reduced weight. The bores don't have coolant passeges between them which makes running lean a dicey practise.
For mild to mid HP street applications they are fine, just make sure the steam holes have been drilled in the heads to relieve some of the pressure in the block. Or just run the heck out of it and see what happens...if you decide to get serious performance it is probably best to go with an aftermarket block.