The 250 blocks are probably stronger because they are more rigid and shorter, making them less likely to flex, plus having 3/4" less stroke reduces a lot of block stress. High boosted engines are much more abusive to components than you think, they have much more cylinder pressure than you can ever make with a N/A engine, and that is what tries to split the blocks apart. These engines are weak between the cylinder because of a relatively large bore and close bore spacing, so there isn't much meat between them in the front to back relationship, plus having a thin deck doesn't give them much strength. Harry's 292 is a good example of that.
The HP potential obtainable today exceeds what a stock block can handle, there have been many block failures to show that is a fact. You wouldn't use a 1966 Chrysler Hemi block to build a 8000 HP Top Fuel motor. The warning signs are waving all around, luckily no one has gotten hurt yet, but keep throwing the gas on the fire and someone will.
