Simpley put. Buick Motorsports introduced a line of blocks, cranks, rods, heads, and all that kind of cool stuff years ago for heavy duty use. The blocks featured beefier castings, Stage I block were basically a beefier "stock" block. The stage II block was built from the stage I block, and features 4 bolt center caps, 14 bolt head provisions just to name a few key items.
There is a lot of little tricks done to it and the oiling system. The on-center 255000153 blocks began like the off-center stage II blocks, but ended up with cross-bolted mains.
All this stuff cost's a bunch, basically because everything is custom.
The Buick power source book really answers your question in detail. I don't have mine in front of me right now.