I would highly recommend that you NOT use that circuit. As far as I can tell, it is a injection-locked self oscillating system. Frequency locked systems are NOT phase locked, and you will have a big input to output phase error, and it will be temperature and power supply dependent!
Since you are at 60 Hz, I would just use two DC crossing detectors (high speed comparators), and drive a set/reset type counter, that counts a higher frequency signal. Something like a stable 1 MHz signal, gated on/off with the two zero crossings. Your count is proporitonal to the difference between the two inputs.
You could use a cheaper circuit, like a ramping signal on a capacitor started by one zero crossing, and a sample and hold sampling the voltage of the capacitor at the other zero crossing. Voltage sampled is propostional to phase difference between the two inputs. You need to drive the capacitor with a current source to get a linear ramp.
Rich