Head Rebuilding/Installing 117

[Strife]

A 2-Parter:

1. I have a 560SL and I'm thinking about getting a 560SEC for the winter. The SL has about 97K, but the SEC will have more and no doubt will need a valve job in a few years. I intend to keep both cars long-term. I read the 107 manual section concerning some of the slightly exotic techniques required to install the valve guides/inserts (involving liquid nitrogen, fer gods sakes). But given that virtually all engines now have aluminum heads, don't ALL modern engine heads require these techniques? So shouldn't any rebuilder of modern engine heads be able to do a 117 head?

2. I've installed both V-8 cast iron and 4 cyl aluminum heads on cars before (and they racked up a lot of miles thereafter), but the helicoiling thing on a 117 really has me worried. I want to do this right, and I would do helicoling(because I know, that as the sun rises, the very last bolt on each side that I final torque during installation will otherwise strip it's thread). I'll even buy the guide plate if I have to. But is this actually doable (like, without a really, really expensive "oops") while the engine is in the car. The manual alludes to a 90 degree drill adapter. Has anyone done this personally? I REALLY hate to take an engine out, particularly one with about 200 wires connected to it. I will also have space limitations.