Dev
I had been to Kullu/Manali in 2000. Traveled by bus from Una (Himachal Pradesh) to Kullu/Manali. I don't remember much of the journey but it was quite scenic. I'm very doubtful about the 6 lane thing - is it possible to have 6 lane roads in mountains?
Have you traveled to Ooty/Kodai by road? Are you prone to nausea while traveling by road in hills (more likely on the way down for those who are). The food in the dhabas along the way is quite tasty but you might want to avoid it in case you are anxious about getting sick.

P.S: Don't miss the trip to Rohtang Pass.

http://wikitravel.org/en/Manali