When I tried changing the glow plugs in the 87 I had w/o removing the intake, I managed to break two 1/4" swivels since they were the only ones that would fit. I started with #6. I actually got it out. Threading it back in was even harder. It didn't quite go back like it did and I probably cross threaded it.
Anyway I took the manifold off after that to avoid any more potential disasters. With the manifold off I finished sinking the GP in #6.
It was a tough last few threads.
With the manifold off the rest weren't too tough to replace.