I dont particularly like going up thru loch lomond the road is narrow and the surface isnt great with at times lots of tourists buses. My preferred route towards inverness is when heading towards glasgow is to go to greenock and get the ferry over to dunoon, head to inverary-oban-ft william-drumnadrochit-inverness. Misses out the a9 as well. Just my opinion. Not sure what your accomodation plans are but the scottish youth hostels at inverary, oban and ft william are all pretty good