I would think there are several options. If you took the head to a machine shop, they could install a heli-coil or timesert, or possibly build the area up and re-tap the threads. I think the "landing" where the spark plug rests is what will determine how far into the head the plug protrudes. Be sure the plug size is correct, and that this landing isn't lowered any, letting the plug drop too far into the cylinder. I doubt this is the problem, however, as re-tapping the hole wouldn't have changed the height of this surface. I'm also reasonabley sure it wouldn't have run for a few hours before smacking the plug if there were interference to begin with.
All this is just a semi-educated guess, however!