I'd say find a base near Drumlanrig, I'll ask around, what kind of accomodation are you after? From the near Drum base take in Mabie and Dalbeattie one day, Kirroughtree the next and finish off with GT before heading south again. Sure theres good and arguably better cycling all over but this would maximise biking and hitting the more famous bits. Some of us should be around to hook up I'll definately try t get another one of those days.
Another option is a big ride out of Drumlanrig, I'm hoping to recce this some time this year and if any visitors are staying and want to ride after the national I'll try to organise it. Both myself and Gazz are putting plans together for the Drumlanrig meet and a day after ride has been mentioned. This is meant to have a crazy singletrack descent. The ride is detailed in 'Bike Scotland: Tails guide' Called the 'Wanlockhead gold rush'.