66 MHz isn't a particularly high frequency, so you probably don't have to worry about any form of signal attenuation other than that due to the resistance of the inner and outer conductors. Some 50R cable can be quite resistive, so it might be worth checking out the specification of the cable you have in mind.
50 feet of cable is roughly 75nsec of propagation delay, or six wavelengths of your 66MHz clock, so it would be a very good idea to terminate the cable at one end or the other. Terminating at both ends does absorb any residual reflections twice as fast, but for a simple
66MHz clock this isn't really necessary - all you need to do is make sure that any reflections aren't big enough to cancel out your clock signal at the receiving end.
PECL-style outputs are emitter-followers, so they need pull-down resistors to sink enough current to keep the output transistors turned on and linear over the full signal voltage excursion, which is easier to arrange with the terminating resistors at the receiving end.
Hope this helps.
----------- Bill Sloman, Nijmegen