They say a 2 bolt block is good to 500 hp if you use studs on the mains. Forgot where I read that. Ah now I remember it was a smallblock chevy book by Grumpy Jenkins.
I had a 2 bolt 400 block that I was going to go with but the darn thing had too much wear on the outside of the bores, actually found two 400 blocks with the same problem. Yes the 4 bolt 400 block is prone to stress cracking the main web area due to the material removed for the outer bolts on the caps. I ended up with a 4 bolt 400 block after all, haven't had any problems yet.