Mike,
I think I would just take them out and not worry about the small amount of strength they may add to your chassis. Your car SO strong from the cage forward, which is the most important area, that the additional strength or stiffness around the engine is somewhat redundant. If, God forbid, you ever do a really big crash, the cage forward is what is important, if the chassis from the cage back literally broke off, which it probably wouldn't as it is a very strong design, you would still be safe.
Rex