The bigger you can get away with the better. Not that it's going to matter on these motors, but when you get into really high compressions or above atmospheric pressures you can increase the energy released by your ignition system and then increase your plug gap and give the mixture a better chance at a complete burn.
Longer gaps take more time to jump, and the spark generates more momentum crossing them, so you get a better burn. This only matters with ultra high performance set-ups. On a 235/261 with 9.5 or less compression the stock rating for a hei (.045"),if you have one, will suffice jes fine.
