with this harness you will be able to cut off the end that would plug into the ECU and use it as a pigtail to tie into the MS3 harness... now if you want to save money you could then pin the rotunda harness to the ms3 connectors, but this requires some soldering skill...
If you tie the rotunda harness into the MS3 harness you can then leave the extra unused cable tied out of the way under the carpet for future expansion.... like if you decide you want to do LS coils and such then you can just take the needed wires into the engine compartment and wire away.