[bigeasy1]

I assume you mean a 350 C.I. multi port injected I/O.Not many of us I/O guys.

I'm not sure of the correct answer,but when I wired my 350(an older one)I only ran the 12 volt gauges and ignition off the wiring harness.
I ran separate wires from the battery switch to a couple of + & - blocks in the stern,and ran my grounds for the stuff in the stern like tabs,pumps,downriggers,and blower off of them.
I ran separate + & - wires off one block,to the front where I have a panel at the helm with eight switches,for the remaining stuff like lights, electronics,tabs,wipers,spreader lights etc.
I think it may be a bit easier to trace any electrical problems with a dedicated 12v supply.In my case the wiring harness is tucked up into the gunwale where it would be very difficult to get at,if I had a wiring problem.

I ran #4 gauge tinned wire from my battery switch to the blocks.I also ran #4 gauge positive and neg.wires from them to the helm.
I used mostly 12 or 14 ga. wires to each item and switch at the front.