Adding Rally type or aftermarket wth tach guages wiring
 

If you are adding either Factory type or Aftermarket appearing dash cluster do you need a new dash wiring harness or do you just add a wire for the tach trigger.

I'm doing that now. I bought a new harness for Rally gauges and the temperature and oil pressure sensors. I currently have aftermarket mechanical oil pressure and temperature gauges.
I haven't started the project yet. Just got the parts yesterday.

Thanks! Keep me posted. I am going to run an HEI and getting an engine harness for that and a internally regulated alternator.

I did what you're thinking about doing. I bought a repo Rally Dash gauge set and there is an adapter harness for that I used, works well. The gauge harness is what the factory used for the rally gauges. I did the HEI / SI as well using a conversion harness, happy with both. They are M&H I bought through our sponsor. If you plan a disc brake conversion, then you can use the unused blue oil pressure light wire to connect to the brake warning light terminal on the disc brake valve assembly. The unused oil pressure light for the old dash can then go into the Brake warning light hole in the rally dash, it's there just not obvious. One step further, if you can find a parking brake switch & bracket or fashion one, you can tap that into the repurposed oil light for a parking brake light.

new harness for 65 GTO with Rally gauges:
 

The instructions for the adapter harness. This section of wires piggybacks alongside of the existing dash wiring in the car.

For 1965 Lemans & GTO this is how they decided to do the dash wiring; if the car was ordered from the factory with rally gauges.

Just FYI. Back then you could only get the tach & gauge package by having the car built with at the factory. No dealer installed gauges.