tach that will work on 6.4 with factory computer/wiring

So I have a fairly old Auto Meter tach on the duster now, you can adjust it for 4cyl,6cyl,or 8 cyl, only way i've seen to make it work is to get an adapter that has to splice into all 8 coil wires. Don't want to do that. Was looking at some of the speedhut tachs would they work with say a msd gmr hookup and setting the tach to .5? Any advice from any who have already been down this road would be much appreciated. Thnx