4 cyl. tach conversion

My experience with a 1988 Omni turbo. Tach needle was driven by a stepping motor. That can been determined if needle stores RPM, when engine is turned off. I made my own ECU with direct fire ignition. It was waste fire,which so I needed to generate tach drive signal with ecu, and switched inductor to generate 60V pulse per ignition event. It worked great.V8 tach may be easier solution, unless you can design a high voltage flip-flop to divide V8 triggers by 2.