I know this has been discussed before but for those that missed it, Lowclearances.com has a huge database of low clearances and it's growing. It can be used on your computers mapping program and on your trucks gps. They are running a special now, lifetime for $14.97.
http://www.lowclearances.com/index.html
Your membership allows you multiple downloads for all your devices
I am not involved with this company in any way, just think it is a good product, especially since some of the new montana's are so high