Hi Steve,
It's not as hard as you might think, especially if the engine is out. I replaced mine with both engine & body in place so it can be done. With the front cover plate removed you can thread a length of tubing into the T section and pick it up through the access holes in the top of the central spine. If the gearshift is out of the way then you have more room to wave your arm around to find it, but even so it's not that difficult. Then you just guide it through the rear holes into the engine bay, in fact if both the engine & gear linkage are out then I'd guess you can dispense with trying to find it from within the cabin.
The downside to this method was that I had to make flares in-situ. You can do the first one (m/cyl) of course but unless you're a demon with a measure then you'll have to make the rear flare once you've got the tubing in the engine bay.
Alternatively you could just run it externally from front to back, fixing it at intervals underneath the central spine. The Elan did that originally, or at least mine did and there's an argument that being able to see the pipe throughout it's length is better than having it hidden away and potentially rubbing on something inside.
Brian