I assume the New Holland is a NV140 or a TV6070 etc? Is the flail mower; an arm type hydraulically driven or coupled to the 3 point hitch and pto driven etc? An excavator and a tractor are two different types of machine. If you have many miles of continuous shoulder mowing a tractor or wheeled machine is what is usually used. If you are just doing short isolated sections of roadside then an excavator can do that job.
Discuss this in the forum topic Brush Cutter?.