Yellowstone to West Glacier

Hard to believe the word “best” could be suck a generality, I know….but yet people you don’t know are supposed to know what you mean by “best.”  Lol

If you made it to the middle of YNP from….wherever….with that rig, you’ll make it out any route you choose. Although the Beartooth Highway has some nasty switchbacks. And may restricted at your length, but that’s way out of the way anyhow. 
Both other highways north to Emigrant or Big Sky are fine. 
Translation, you have 2 choices that generally head in the direction you need to go and both are fine. 
Or was there a different question you were hoping we’d glean by telepathy?

I  am making that trip in early Sept.  I'm stopping in Butte, Mt, but my plan right now is to go to West Yellowstone to 181 to I*90, (Butte), and then I 90 to 12, to 141, to 200, to 83, to 35, to 2.  I'll follow this post to see what others offer.