I've never cycled these roads but I know them well. I used them frequently when I was out on the motorbike, when I did the the Police motorbike training and my colleges and I are often sent to these roads for cyclists and motorcyclists.
These are all fast roads and often driven as such, the only section I would say that people do not drive at high speed on is the Aberfoyle to Killmahog and that's because the road is so twisty its hard to get any sort of speed up. Unfortunately that means lots of blind bends though. I wouldn't fancy doing the A82 south of Crainlarach, it's a narrow road and its the main road from Glasgow to Fort William and the north west. There are often buses and HGVs' squeezing (and I mean squeezing) past each other as there is hardly enough room for them to pass.
I would point out though that the scenery will be outstanding, and with it being in April hopefully the roads wont be too busy and as pointed out, its for the experienced rider who knows how to deal with fast traffic.
On the other hand I think the 65 mile route should be great, the loop round Loch Katrine should be very quite.
It looks like half the route is on trunk roads!!
