One great way to link the TA and the NT out west is to take the Great Parks North route from Missoula up to Columbia Falls. It's worth a few days to backtrack to West Glacier and Apgar to see Glacier N.P.
Great tip there. That looks like a better option than trying to do the Divide trail on a road touring bike. I'll probably miss a few views, but Missoula is just as good if not better than Helena.
Wouldn't to the Divide on 32c/37c.
I will be going back to Glacier N.P. again this June to ride up the west side of Going to the Sun to Logan Pass and back down again. This will be during a loop tour from/to Missoula. Here is the entire route:https://ridewithgps.com/routes/19192705
The latter miles show the link between Missoula and Glacier, only in the opposite direction you would be going. Much of it is part of ACA's Great Parks North route.
There is camping in/around Seeley Lake and further up MT 83 at Lake Alva. (One of the two campgrounds there has no water. Lake Inzez Campground is also dry.) In Big Fork, Wayfarer's State Park now has hike/biker sites. Note that getting to and from W. Glacier involves some dirt at mile 428/511 on the above map unless you want to brave a shoulderless section of U.S. 2 going towards Hungry Horse. 32c shouldn't be a problem, and it's pretty and quiet back there. The pavement picks up at some point at the easterly end of Belton Stage Rd.
In the park you could set up camp and Sprague Creek Campground or further up the road at Avalanche Campground and then ride up to Logan Pass and back. Sprague Creek is smaller (nice tent pads and bear boxes for hikers/bikers) but it's also close to the road. I like it there because you can walk to Lake McDonald Lodge, which is a must-visit. However, if you stay at Avalanche you have a shorter ride up to the pass and you can catch the free shuttle if you want to go the lodge for dinner and/or a beer down by the lake. Personally, I don't like to be dependent on shuttle schedules.
In Whitefish, on the NT, Whitefish Lake S.P. also has new hiker/biker sites with lots of amenities. Lots of train noise though, so bring ear plugs if you are a light sleeper.
BTW...If you want to take what is supposed to be a very nice detour off the Northern Tier, take a look at the section of the above map between mile 255 and mile 339. (You would, of course, be going in the opposite direction.) From what I have read (including a journal on Crazy Guy), it's supposed to be very nice. You can see the portion between mile 255 and Yaak at mile 297.4 on Street View. There is camping in Yaak itself at what appears to be a lodge/restaurant. There are also a couple of forest service campgrounds with water west of Yaak at mile 292 and 294.5. There is also a store in Yaak, but I don't know how well stocked it is. Closer to Troy, there is a campground at mile 260.7. Troy has a legitimate grocery store. Really excited about riding this section. The journal entry I read says the climb from Lake Koocanusa up the summit at mile 226 is a tough one. I will be going in the opposite direction. It looks like that way is easier grade-wise.