March should be fine, only a little colder, but weather could be more stable, your real enemy is wind, rain will be a given any time of the year, cold does not set in till later and never actually that cold You know real cold in Ontario. Which part would you do?, some parts are wetter than others. I have driven most of it at various stages, never cycled it. Gorgeous, but tough. Very coarse gravel up to rocks on the surface, don't plan on 120 km days, think of 60-80, wide tires, and take a good look at how you fix the racks, everything shakes loose when you rattle downslope, a trailer might be a better idea. If you do go with racks, I never liked the 5 mm bolt that attaches the rear rack to the dropout, put a 6 mm if you have material to thread a larger bolt. A sheared 5 mm bolt is very possible on these roads.
Highly Recomend 26" wheels in order to find replacement tires if you need them.