The long plugs don't reach in far enough to hit the piston, if you do use those plugs the engine will still run, but it will not run as effieciently because the plug will protrude into the dome shape of the combustion chamber. O.S. #8 is probably the closest to the Go #5.

Jumpjunkie, the engines come at the minimum shimming for 30% nitro so performance should be close at altitude, after break in (1-1.5 gallons) you may want to pull the thin shim out if you feel you need more power.