[jonnyk]

Once you get beyond about 500 rwhp the stock block will split up the middle EVENTUALLY. The B50 block is stronger yes, but in the wrong areas IMO, it's the bulkhead in the lifter valley that is the weak point and they don't add much/if any meat in this region. Some guys have split the B50 already too. Because of this, I don't ever recommend a main support because they aren't helping the weak spot, and are only good to hold together the block once it splits. Your best bet is an R block or Dart block for serious power.
As far as RPM's are concerned, I haven't personally heard of a case where main cap walk was the reason for engine failure...I wouldn't be hesistant to spin an all motor 2 bolt 302 to the moon at all. A set of main studs is good extra reinforcement as well.