Hi there,
I ride similar gearing (71) on mu longish hilly commute. I also ride bullhorns, profile designs with 5cm drop and TT brake lever - left, as I am (edit: was) riding road bike as well. My bike has very short (for my size) top tube, and I ride 120mm road stem. I ride the tops only when I want to streach, so for fast rides the brake lever is exactly where I want it.
Fine tuning would be cutting the 3cm tips of the horns, as I am never that far - when I want to go more aero, I just bend the elbows more, with hands about 3cm below the tips of the horns. Cutting will allow to reach brake comfortably from the position just below the bends, this is my relaxed position.
You can see this setup here:
http://www.pbase.com/wojtek_burkot/biking