It's the 556 east of the Searchmont resort. It's also called Ranger Lake Road. That's close to the Soo and it meets up with the 129 north of Thessalon. I doubt it will get paved in our lifetimes. It's pretty twisty for a northern gravel/dirt road, but totally doable on an 80/20 tire, because it's not too soft. I did it using a TKC80 on the front and a Metzler Tourance on the rear, with both tires aired down by 10psi. There is a gas station where it meets the 129 were you can air up your tires if you don't have a portable pump.
I really liked it and as a gravel noob, I was pretty nervous at the start, but by the end my gravel riding skills had improved a lot. I also went with a guy who used to race motocross that helped.