Mike,
I used a piece of .030 aluminum sheet to modify a number of MTH boxcars.
I turned the metal floor over, and cut the aluminum to fit each end of the car. I drilled the hole for the truck and the 2 holes for the screws that attach the floor to the body. I then shimmed and super glued a Kadee coupler to the aluminum plate:
The aluminum is much easier to cut than brass, but you've been given a lot of good possibilities for cutting brass. I use sheetmetal shears for rough cuts and a Dremel for final shaping.
I've been using these aluminum plates at least since the 3RS sub-forum took off and possibly before then, with no problems.