[RACER X #99]

Thicker cylinder walls, thicker bottom end main bearing area, dual external oiling feeds. If you are just building a street engine you probably don't need this. If you want a serious HP small block 500+ hp then you need the 4 bolt mains. The stock 351 is a very weak looking block when compared side by side with the Dart. Even the 4 Bolt Chevy block looks weak compared to the Dart.
That's why I bought one. Plus only about $500 more than the SVO block.

Cranky