Green2384

I recently did a swap from a 2000 350 mag mpi to a 07 496 ho with smart craft. The tach used to work on the 350. I've been told two things. 1. I need to switch the computer from digital to analogue. 2. There is a wire for and analogue tach can complete please give me the correct answer. It's driving me up the wall because I can't find the extra gray wire. All my other gauges work find and the gauge does get power. Any help greatly appreciated.