First thing I would do is to use a thread chaser. There are some that are made specifically for spark plugs. When using it, proactice in a hole that is not cross threaded so you get a feel for the angle. You don't want to fiddle around trying different "paths" as you will certainly ruin what threads are left. Use some light oil, like 3 in 1 when you chase the threads. If this doesn't work, then you need to use the heli-coil. NAPA has kits that are specifically made for this. Unless you take off the head, you will get some aluminum in the cylinder, but I don't think this is worth taking the head off. Here's my recommendation for that - bring the piston to TDC. After installing the heli-coil, attach a fuel line hose to a shop vac and insert it into the spark plug hole and vacuum out the cylinder. Anything that is left in the cylinder will get blown out when the engine starts. Aluminum is soft and probably won't scratch the iron block.