I'd take a careful look at some of these - you're missing a couple highways that are fully signed, like Montana 67 and Montana 68

Feel free to check out my site, if you'd like - it's several months behind but I have what I feel is a pretty accurate route log, backed up with photos. If you need photo evidence of anything that I don't have pages up for, I'm about 95% of the way done clinching the primary system- I think I'm just missing 66, 67, 80, and 81, (just haven't gotten all the route pages up), so likely have some photo evidence of anything saved on the hard drive and just haven't gotten it uploaded.
http://corcohighways.org/?p=32There's other highways that I'm not sure how you'd want to account. What I consider to be MT-65 and MT-88 are kind of in flux- these don't have signed numerical designations but are clearly state highways. The internal number in those cases is P-65 and N-88, and the MDT Route log actually references "Mont 88" in one place. That being said, they may just be unnumbered.
Other highways like Montana 47 arguably follow the business loop all the way around Hardin, and don't terminate at the interstate. That's supported by internal route log and some field signage.

I agree with ending Montana 2 at I-90, instead of having it continue through Butte - while that appears to be the case internally, there is no field signage to support it.