50 Ohm TTL Drive Strength
From page 3-13 of the data sheet you can glean that the front panel inputs have a programmable termination of 50 Ω to ground, which can be used or not. They also have programmable voltage threshold, so if you want to drive at less than 5 V, you may.
For the lowest drive strength requirement from you box, use a series 50 Ω output impedance, to an unterminated 6672 with a 1.6 V threshhold, and something like a 3.3 V drive voltage. This will require a brief 3.3/100 = 33 mA burst of current, while your output drives the line impedance in series with its output resistance. After a delay of one cable length, the 6672 input will rise to 3.3 V, and after two cable length delays, the output current will drop to zero. This series termination style is only suitable for a point to point signal, as only the output see a clean 3.3 V step, the rest of the cable sees a brief hesitation at half that. The series termination at your end absorbs the reflection from the open circuit output.
If you want to use the 6672 terminated, you can still use a series termination at your end, but choose a threshold voltage of 0.8 V. The output will still source 33 mA, but the 6672 input will only reach 1.6 V, and your driver current will stay at 33 mA. This termination at both ends of the line gives you the largest margin of protection from reflections.
You could drive your 50 Ω cable directly, but then you must select the termination at the 6672, and the output current will be higher, 66 mA for a 3.3 V step.