Ok, more thinking. The "U" brace may work but it is a lot more work. I think what I will do is anchor the outside portion of the motor mount to the chassis. Currently the motor mount is only held to the chassis by the two inboard screws. If I anchor the outside part of the mount that will eliminate all side-to-side flex, and still allow some up and down chassis flex. I also have a small press and a micrometer, I will put some pressure on the center of the chassis and see where/what flexes and by how much. Crude, but I guess it may provide some idea on the flex points and how flex I need to deal with. A small hard rubber dot/bumper under the motor close to the mount will provide additional motor support if needed. Please forgive the crudeness of the drawing.
Front view of the motor mount. Normally mounts to chassis with two inboard screws. Want to add one to the outside to reduce flex.