I rode out of Glasgow up to Arrochar with my daughter a few years back- they had not quite finished the Glasgow end of the cycle route and we ended up riding a mile or so through some bits of Glasgow I would rather not have visited- but I think we got lost a few times and there was a distinct lack of signs and places where the map indicated route was blocked by building work. I have been informed that the Glasgow end since is mightily improved- might be worth checking with a cycle forum based there- I only know of GFGSS, but I could check with my daughter once she is back down from the mountains next week. (Although I'm betting a YACFer will post from there sooner or later). I remember the next bit of the route being fine, through Dumbarton then alongside the Leven up to Balloch and then on to Luss- after Luss (if my memory serves) it becomes a track right alongside the A82- safe and functional but not pleasant. We were not enjoying the ride at all from here (our original plan had been to head for Crianlarich ) too many huge trucks whizzing past- (the noise of them was phenomenal and sudden because of a massive headwind) so we bailed and took the train from Arrochar and Tarbet. I don't know if the cycle track carries on alongside the main road after this or if you have to use the road.
I would agree with Tigerbiten about time of day. I have ridden the A82 all its length at different times- early morning it's a great road to ride, little traffic, easy to anticipate the arrival of the occasional big container lorry and awe-inspiring views. Other times, when it's been busy with lots of large trucks, I haven't been able to enjoy it so much- it's just been a means of getting from A-B. If it were me, I'd stop overnight in Arrochar- start really early next day and head north taking a detour up Glen Orchy (yes it's out of your way but it's such a pretty road and so quiet) and then stay in Bridge of Orchy area for another night (that would be about 40 miles). Up again next morning early and head for the Corran ferry.
However your Arran Kintyre idea is also a good one. You could do some real island hopping and go from Kintyre to Islay then Colonsay and up to Oban that way. I am planning something like that for next summer's trip (Dungeness to Durness).
As for cycling through Glasgow- well, my daughter reckons it's easier and less pressured than London- however there are lots of one ways as I recall and the couple of times I have done it, I did get lost although still managed to get to my destination eventually. Depending on when you are travelling, it might be possible for me to get Frances to guide you through the city.
It's a great trip you are planning, have fun.
