I am going to attempt to re-purpose the wiring loom from my donor Sierra as much as possible.
I have found the following which my uneducated eyes suggest is a useful source of info - as it looks like comprhensive wiring diagrams for the Sierra: http://jeankule.free.fr/Pinto/1982-1989.pdf

Is there any key info I need or sources I should be looking at for info?

I have seen mention of 'switched earth' and 'switched live' in others postings about wiring - is this something important that I need to know about (i.e. did Sierras have one and then the other based on what year they were built or something...?)

It's all about the column switches. IIRC, early Sierras switched the live to each circuit directly, whereas later ones switched the grounds to relays, which in turn switched the lives to the circuits.

If it’s any help I’ve got pdf copies of the Sierra Haynes manual I could email to you.
Drop me a pm with your address if you need them 👍