I went the old A9 on my LeJog specifiably so I didn't have to cycle the A82. From Glasgow I went Callander, Killin, Pitlochry - old A9.
I've driven the A82 between Glencoe and Glasgow hundreds of times and cycled short stretches of it. It isn't all bad. From Balloch to Tarbet the road is modernised, 2 wide lanes so cars can pass easily sharing the lane. As stated there is also a cyclepath option with a decent surface.
Tarbert to Inverarnan the road is so narrow HGVs come to a standstill to pass each other. It is mainly so twisty that cars are doing 30-40mph so taking the lane is feasable. Inverarnan to Tyndrum fairly wide lanes.
Tyndrum to the far end of Glencoe - narrow road with many long straights where cars get up to motorway speeds. The lanes are too narrow to safely get 2 cars and bike side by side so you are relying on cars doing motorway speeds judging their distances and gaps and being prepared to brake from 70mph to cycle speed.
If the distances could suit I'd suggest an overnight at Tyndrum followed by an early start to get to the top of the hill in Glen Coe before full holiday traffic builds up. A mirror to watch the behavior of traffic coming up behind would be useful.