Without a picture, it's difficult to imagine. Could you drill through the sideframe and the metal plate (with them properly aligned, of course), then insert a wire "pin" to hold them in position? Probably a hole and pin at both ends of each sideframe, with the pins ca'd in place. You'd have to ensure that the pin was flush on the outside so that it wouldn't be noticeable, and flush on the inside so that it wouldn't interfere with the wheels. If the metal plate is for electrical purposes and to allow the wheelsets to equalise, it might be better to use only a single pin at the centre, length-wise, of the sideframe. In either case, I'd use music wire to make the pins - use a cut-off disk in your Dremel to cut them and to finish them flush.
Wayne