Terminator X Hole Through the Firewall

I've looked a little bit into this one before on a different project. I don't know about your '73 but the pass through on the driver side for the foot washer pump or speedo cable looks like it may be a viable starting point for this setup as a pass through if your going to use the Holley dash setup.
Holley Terminator X & X Max Firewall Bulkhead Grommet - Universal