You are talking about the metal pipe that runs from under the clutch housing, along the front of the subframe and disappears under the rad ? Not simple.
The first time I did one, I asked both the local BMC workshops, and they both said "engine out".
Needless to say, I "cut a hatch", and then had to make it bigger, but I got it undone.
To replace the new pipe, I cut a slot in a box spanner, put it over the union along the pipe and held it from opening up with a jubilee clip, which got my spanner to somewhere under the starter pinion.
The ******* union cross threaded as I screwed it in and I had to take the engine out, but otherwise, I recon that the box spanner idea would have worked.
In 40 years, I have only done about 4 since, but as I have the where-with-all to extract the engine, I don't think twice about what I'll do.