The switch wire would most likely fire the relay to power all the pink leads going to the PCM.

Yeah, do the 8 gauge since the fans seem to be powered by that fuse block.

About all you have to use from the original harness is the switched power wire from the keyswitch and the purple starter lead. The rest can go away except a/c leads if you have them. Oh yeah, save the oil pressure, water temp and alternator leads, too.