If you continue on past Carnwath, carstairs then Hyndford Bridge on the A70 there is a great, but short stretch between there and the M74. Past that and continuing on towards Muirkirk is good too.
One thing about these roads during weekdays there's masses of Coal trucks shifting at a speed which almost defies physics at times. I've been heading to some of our sites, not in the scoob but in a company car, to be greeted by a Coal Truck coming round a blind bend in a 16 wheel drift.