My preferred method for adding weight to both resin and plastic house cars is to screw a strip of sheet lead to the top of the floor before permanently attaching the floor to the body. See my article, "Building resin freight car kits," in the February 2010 Model Railroader. The section on adding weight is on page 52.
I like this way of doing it because I like magnetic uncoupling and so try to avoid using steel weights that can be attracted to magnets. I routinely discard the steel plates included in many kits and use lead sheet instead. If the holes for the screws are drilled between the center sills, the screws will be invisible on the finished car, and with screws there's no delay while glue dries or concern that glue will eventually shrink and come loose.
Good luck with your car kits,