I think I know what you are talking about...
simple weight saving mod that I did to my car.
look under the car... and right under where the engine is (almost centered) is a piece of metal that goes across part of the frame. You should see it it's pretty obvious and is about 6"x6" it's held on by a couple of bolts and is quite heavy for its size.
I may be mistaken but scoobymods may have a DIY on that too.