The gap that plugs come in doesn't matter. That can be changed with a gap tool. Its a little piece of metal that has both sides angled, one usually from .020-.045, and the other side from .045-.070. Just slide the tool between the curved metal and the base post up to whatever gap you want. Most tools also have a hook on it to bend the gap back. Basically all you are looking for is a plug that is slightly hotter or colder than stock, depending on what you wanna do. On my 69 GTO, with GM HEI ignition, I used the R44s with a gap of .047 , but the stock plug was R45s w/ gap of .035, Ram Air IV used a R44s w/ gap of .035. It all depends on what your specific engine likes. Just buy the tool, and try a bunch of different gaps.
A quick guide:
Bigger gaps= more spark travel (better combustion burn), but can get covered in gunk easier
Smaller gaps= less spark travel(worse burn), and less chance of gunk
Just find the happy median.