If the dirt roads are on CN you should be able to make most of the tracks work for you on a Zumo 550. I have converted large track files to a route IF IF they are in the CN steet db.
While not a "track" machine, the Zumo 550 does convert tracks to a route but in my experience one can experience problems when moving from pure dirt to mapped streets, especially if tracks are very close to a mapped street. Best to check the results on MS too...indeed MS is needed for some of these machinations so this is an exercise in trip planning prep not for the sticks.
Loaned my Zumo to somebody so can not test UTBDR tracks now. For pure dirt (unmapped) sections you may have to break some of the tracks in two as there is a limit of about 250 via points in a converted route. Based on how the internal Z550 Algorithm "magic" works on the filtered track file may be cut down even more in the conversion to a route.
One of these days I will get a Montana but until then I guess I am in to work arounds.