It was the toe-in. And coming from the Big Honkin Hammer School of Technology, I fiddled around with the Kool Stop Salmons for about ten minutes before my patience wore thin.
The solution involved removing each pad assembly from the canti arm, clamping the threaded post in my trusty Harbor Freight big arse table vise, and leaning down on the pad to insure that the front would make contact with the rim first. It took two tries, but now there is just the pleasing swish of the pads doing their job against the rims to bring the Trucker to a perfectly modulated and purposeful stop.
No calipers, no levels, no torque wrenches, no problem.