The word I got from NHRA was that the 400 block is a legal swap for the 440. (SS only) Dont think you can do it for another combination unless you get special approval. They must maintain the original 440 main size to be legal. If you want to have a smaller main you just use a bearing spacer.
ALL SHORT DECKS are LEGAL (Super Stock only) if you use the approved block for your combination.
Man are they going to see some trapezoidal engine combos.