Give me a few hours or days. Or you can look for yourself at the GM performance site, actually i think now called Chevrolet performance:
Ok, i'm obsessed: i think it's page 157 in the catalog #12480157 block (i was going to build one), then they also have 2 aluminum blocks
http://www.chevrolet.com/performance/overview
i think Dart has one, but i don't have time for that.
Also, i think Blueprint has their own casting (?), but unfortunately i have heard too much negative comments about them.
But the bottom line, for me, is that if i was going to build a 400 (or 400+, 421 etc) i would start with a new block. Because think about how much time(and money) it will take to track a used one down. Then you have to check it out----and what if it's cracked? At the very least it will probably need to be rebuilt. The aftermarket block are also mucho stronger than stock.