RE: An Expensive mistake.
Yes, the head and cylinder are cast as one piece (usually), and generally referred to as a top end. As with a nitro engine, most of the cooling happens above the plug, but gassers dont have a tall head, the cooling find are cast around the sides of the cylinder. Some aftermarket cylinder kits actually have a head with a head button that is detachable from the cylinder, and some have a "wrap around head", which pretty much just extra fins you bolt on top of the cylinder. At the base of the cylinder are the holes which allow you to bolt it to the block, some have two directly across from each other (2), and some have one on each corner (4). The plus side to 4 bolt heads, is that they seal a lot better, and your gaskets are less prone to blowing out.
I really dont understand why all of them aren't 4 bolt, it shouldn't be an upgrade, it should be standard.