You are supposed to put very thin washers (they come for that purpose), but I use to change sparks to different holes until they fit, and buying more than six at a time. Sometimes it also takes different torque to get it right (not too much nor too little of course).
It is supposed to be looking at the intake valve (the open gap), just look at what side is the injector to know it.
It would be a good exercise to do that in the dino to see how much it gives, but it is an old trick and even if it gives 1/4 horsepower it is absolutly free (except for time)
Indexing is considered just part of the detail of tuning as far as I am concerned. You open up the flame front to the intake charge. This means open side of the plug to the intake valve. There are several sources for indexing washers. Jacobs, Summit Racing, etc... And I am sorry but multiple electrodes still should be indexed. All that a multiple electrode plug does is take a regular electrode and split it. The flame front should still be faced to the intake valve. How much HP is gained is a matter of which motor it is done on and I doubt anyone ever will check just that detail. My local dyno is $180 first hour and $50 after that. Don't be tempted to just overtorque to get them indexed, you'll pay with your head. Happy tuning.