The previous article stands on the REPAIR you need to do - but this series of posts from a machinist's chat board is illuminating on spark plug threads, and the quality (or lack thereof) of taps and chasers. Note - these guys are mainly talking about CAST IRON heads - with the heads being freshly cast and definitely off the engine.
http://www.practicalmachinist.com/vb/general/sparkplug-tap-question-196315/
And here is a tool that reams from the BOTTOM UP ... again, not recommending it for the situation and repair you are going to do.
Not vouching for this tool - just stumbled across it - interesting tool. Don't know if it's worth squat or not.
http://toolmonger.com/2007/05/22/re-tap-spark-plug-threads-safely/
Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good ...