Personally on my 83 after the manifold leaked for months I went to buy a used manifold and it as $75. I did not buy it because it was cracked on the back side. Instead, I found a header, decent quality and performance pacesetter for $125 at the local super shop. As for the broken stud AND broken tap in the last stud, I pulled the intake and exhaust manifolds off (get a couple extensions and a U-joint for your racket). I then used a cold chisel and punch and pounded pieces of the tap out. After that, I used a micro digrinder, kinda like a dremel with a flex shaft, bt pneumatic, and ground the rest of the tap and stud out. I replaced it with a 3/8 american stud (forgive me) and opened up the header boss to fit. I retaped the head after drilling it out, put it all back together, done. I had only worked on cars for a couple months before on my own, had few tools and the such, and it took about 3.5 hours total. Had it not been for the broken tap, it would have been MUCH easier. I do not know why you take the head off, other than that is what the service manual says. A lady I know had the dealer do it for her, they drained the oil, removed the head and fixed the stud, put it back together and NEVER put oil OR water in it, a Nissan dealer indeed. She drove a VERY overheating 82 for days until she told me and I put oil and water in it. Total cost for her was $400. I do not trust dealers or many mechanics for that matter, I do all my own work, doesn't matter how hard, I will figure it out myself.
Joshua L.
Mechanics is not for everyone.