I had a company in Sonoma make me a dual a/f gauge for $170. Looks like the gauge in the picture except it has dual a/f meter.
http://www.westach.com/images/showcase/3DC2-1-Dual_EGT.jpg I mounted a 02 sensor at each collector. I paid about $50 ea. for the 02 sensors. I mounted the gauge in a old clock housing and now resides in the clock hole in my race car.
74 rsr w/3.4 twin plug.