I don't think that any of this is going to happen behind the gauge cluster, but at the sensors. I am going to have to install an adapter the feeds the tack a proper signal for an 8-cylinder engine, which may go behind the dash.
There are some good threads on this in the 60s section. I think some people adapt an aftermarket tach. I need to do this as well.
Do you know what you are going to use for a speed sensor?