Chevrolet knew it had a legend in the making when it introduced the small-block V-8, and that legend got bigger when the W-series engine debuted a few years later. When the W-series gave way for the Mark IV in 1965, GM set the template for decades of hot-rodding to come. While the four-bolt-main block might have the cachet of being purpose-build for strength, the reality is that the much cheaper and more available two-bolt block is nothing to sneeze at.
The weak point of the 454's bottom end isn't the main caps, it's the block webs that the main caps bolt to. When a bottom end breaks, most often stress cracks start at the bolt holes and propagate through the web; the more holes in the web, the less material the cracks have to work through and the more opportunity for failure. For this reason, the parallel four-bolt arrangement that GM used for high-performance blocks isn't actually the ideal arrangement.
Serious race engine builders will often start out with two-bolt blocks and re-drill them to accept "splayed caps." A splayed cap places the outer two bolts at an angle to the center so the bolt holes go through the web near its base. This greatly reduces stress on the webs, and redistributes it to the block. But all that is really a redundancy; at the end of the day, the second pair of bolts only stabilize the caps. The center bolts actually hold them to the block.
With a decent set of chrome-moly studs and a well-balanced rotating assembly, a two-bolt block can easily hold 600 horsepower. A main cap girdle can take the two-bolt up to 900, because it essentially performs the same stabilizing function as a second set of main bolts without drilling into the block. Past this point, you're looking at splayed bolts for the extra holding power, and then an aftermarket block with thicker webs.
