In general, I mount an Athearn truck to a wood bolster by turning down the head of a #4 wood screw until it is a loose fit fit inside the mounting hole of the bolster. Doing this allows the head of the screw to contact the bottom of the hole in the bolster and stabilize it. Run the screw in just enough to prevent rocking, without binding the truck.
The head can be easily be turned down in diameter chucking the screw in a Dremel or electric drill and then holding a file to the head as it rotates. I also remove the little ridge on the top of the bolster, as you did. I use this same approach with 4-40 machine screws and their metric equivalents. On cars that came with shouldered mounting screws and springs, I remove the springs and use thin washers between the head of the mounting screw and the bottom of the bolster to prevent rocking. I don't like springs because they still allow the car to rock unless they are tightly compressed, in which case they can interfere with the ability of the truck to turn smoothly around the mounting screw. A small drop of Loc Tite will hold the screw at the right position if it is not shouldered and can't be snugged down.
