This is a bit of a dig up of an old thread, but I have, well my Auto Elec done this conversion on my RB20DET NICS, here is a link to the thread on R31 Skyline club, seems to working fine so far, you may be able to adapt it for the RB25. The only reason I went this way is that the old nics engines connectors to the coil packs were getting old and heat affected and gone brittle, and you cant buy the plugs anymore. The whole conversion cost me around the $250 mark. but i would of prefered going the splitfire path costing around $650
Follow th link below to see how members on th R31 club webisite have been doing them, both the single coil pack and the AU Falcon method are on here, might shed some light.
AU Falcon coil pack converson