You dont list specs of the motor, but mechanical will likely work fine. I chose to run an electric pump because I had issues with vapor lock in the SW heat. For reliability reasons, I prefer a mechanical pump. With an electric, you will need a separate fuel pressure regulator. Give Summit a call and they will spec a pump for your motor.
You probably dont need a spacer but some motors gain a few top end HP with them. You sacrifice low end power with them. I would just go with the dual plane you have and see (cheap experiment).
As for the box diagram, MSD has VERY good instructions. Go here and type in your model number.
http://www.msdignition.com/instructiondownload.aspx