About CD Boxes: Bosch/MSD/3-6-8-pin Can someone explain it all?

[cowtown]

Hi everyone,
I'm getting ready to put a '77 930 engine into a '75 car.

I'm currently in the "God, I never thought I'd have to find so many obscure bits and pieces" stage, meaning the engine's basically ready except for some oil lines and electrical stuff.

I'll be using the 930 wiring harness, pointless distributor, coil (unless I go to MSD), etc. I had thought I could use the 75's 3-pin CD box, but it looks like all the other turbo transplant guys I searched on either use the 6-pin SC box or an MSD unit. I'm working reading the wiring diagrams, but can someone explain:

-Why can't I use the 3-pin?

-Why does the 930 have 8 pins when the SC uses 6 and the 2.7 cars use 3?

-Should I just buy an MSD6AL and tachometer unit and be done with it?

-If I go MSD, is there any value to the MSD unit with timing retard (I have the original 930 distributor with one vac line for retard).

-How can an MSD unit control timing retard, when the the distributor makes the physical connection to the sparkplug?