Before you start on the removal, a few days before in fact, thoroughly soak the threads with the penetrant of your choice. I like P'Blaster, but others have used Kroil (I think) and some folks swear by simple things like WD40.
Soak them. Use the whole can if it makes you feel better.
My tool of choice is a deep 27mm impact socket and a 3/4" drive T-bar. Go slow, go steady, don't get impatient and you won't crack the head.
When you install the injectors you're going to run, lightly coat the threads with a little film of antiseize, they'll come out easier next time.