Yes, Mapsource should work. But there is an issue with installing multiple maps, it always names the map file gmapsupp.img when you send it to the gps, so it will erase the previous map that you sent. The solution is to change the file name after you have sent it to the gps. For example, change gmapsupp.img to WyTopo.img after sending it, then repeat as necessary for the other maps.
Mapsource used an older system for bundling multiple maps into a single gmapsupp.img file. This worked well on old devices, but my experience is that it can create problems on a new model like the Montana. So I think it's best to just change the map file names as you send them. New devices can have multiple map files, but old ones required all the maps to be installed in a single file.
Again, keep an eye on the total number of map segments you are sending as I mentioned earlier. Mapsource will show the total number of segments that you select.
Alternately, just downgrade Basecamp and MapInstall. The old versions still work properly.