Massive dig there mate. You should be able to access all studs with the head on. If you're in Melbourne I can strongly recommend the thread doctor to come out and do the job for you, he charges a bit, but gets the job done really well. Cheaper than taking the head off including time and effort. If not in Melbourne call around, there should be a mobile thread doctor in your state. Unless you a really competent and have great quality easy-outs, it's a job to be left to a pro.