Sometimes, the 2 bolt blocks are even more desirable than a 4 bolt block because of the conversions. There is generally no difference between a 2 bolt and 4 bolt block except that the extra holes were drilled, so it is very possible to convert a 2 bolt block to a 4 bolt by drilling and tapping the extra holes. You can drill and tap the holes at an angle for splayed caps, which makes the block even tougher than a standard 4 bolt block. I wouldn't worry about that kind of toughness unless I was running 7500 rpm plus on a standard motor or 6000+ on a stroker.