A two bolt block is good to 500 hp as long as you use a good set of main studs, not sure of max HP without the main studs.
If I remember right this info came from Grumpy Jenkins. He also said that 4 bolt 400 blocks are weak and will tend to crack in the main webs due to the lack of material there because of the outer bolts for the 4 bolt. I know a few folks running 2 bolt 400 blocks putting out over 500 hp with good main studs and haven't had a problem with it yet.
I personally have a 400 block with 4 bolt mains because I couldn't find a 2 bolt block that was borable .030 over. These blocks will wear a bunch on the bottom of the cylinder areas due to those short 5.56 inch rods and that long stroke. I would have prefered a 2 bolt and bought a new set of splayed 4 bolt caps and had them installed, but it didn't work out that way.
And just for the record, someone mentioned that a 377 is a 400 with 350 rods, actually it's a 400 block with a 350 crank.