Nate: I used the brass bar idea on American Limited floors. There is wasn't so much flexibility as mounting the trucks. The sheet steel weight AL provides covers the bolster holes, and the screws are just a little too long. Tightening them all the way forces the weight up, and leaving them looses looks too "wobbly" for me.
So, a pair of 1/16 x 1/4 inch brass bars, one on either side of the bolster holes, works much better, is stiffer, and weighs more. I ACC'd mine, then use styrene strips to clamp them in place, with a filler strip between the bars, and a cap strip across both.