Hi Chilli
I agree with the other coments. The northern half of Loch Lomond road can be tight and need extra care. I would recommend the M80 and branch off just after Stirling and take the Calendar road, then Calendar, Lochearnhead, and Crianlarich. Watch for the bridge in Crianlarich - it a blind bend and you'll have to take the midle of the road. After that it's plain sailing. Have a good trip and let us know how you get on.
Charles