In reply to Flatus Vetus:
As others have said, the M74, M8, Erskine Bridge, A82 is usually quickest.
But I often go M73, M80, A84, as it's not much slower and there are more spots to overtake, so it can be a less frustrating drive. The side of Loch Lomond is very pretty, but a bugger if you get stuck behind something slow.