We machine a lot of blocks in our HAAS 4 Axis machining center and we have seen many problems with the 400 blocks with there thin decks thin webbing and with the big main line and big cylinders makes the webbing weaker the the 350 block and the bigger bores make for a weaker deck then the 350 block as well.
We have gotten far more emails and calls with problems with the 400 blocks.
Any block that comes through our shop is sonic tested and we have seen a few 400 blocks that were not safe for a high horse power application and we have a virgin 400 block that is cracked from the cam tunnel down to the mains.
Don't get me wrong we have machined a few 400 blocks and some have worked out well and some have failed or have developed cracks after a season of running.