indyfabz

I was going to suggest that, but it's not entirely off road. Just did Avery to the Route of the Hiawatha in June. That stretch of the old Milwaukee Road right of way is open to cars. So two or three on my way up. East Portal down to Taft and the NorPac Trail up to Lookout Pass are also open to vehicles. Didn't see encounter any on the NorPac last year but I did see one car and a couple of ATVs in 2017. That side of the NorPac is also is in bad shape in places. I had to walk a couple of short stretches where water cascading off the hillside had washed off the top layer, leaving a lot of large rocks exposed. 32c might be cutting it close if loaded. (Never been on the NorPac between Lookout and Mullan.) You are also on road west of Avery until you pick up the Milwaukee Road right of way again, albeit on the quite side of the river. The old right of way from there to St. Marie is open to traffic, as the web site notes, but it's supposedly very light. (Various sections of the Milwaukee Road in MT and ID were converted to community roads. Some of them are actually private roads.) From St. Marie to Plummer you are on ID-3.

With that said, if the OP is flexible and willing to add some light traffic roads, I think it's a great option. The Hiawatha and the right of way between Pearson and Avery, with it's many tunnels and trestles, is a great ride.