I think you are talking about the rear holddown plate. It sits on top of the rear crossmember of the frame under the floor. It is bolted down to the frame through the rear C channel and gets riveted to the rear outer skin.
I just measured mine it is 41" wide by 3 1/2" x 1 1/2" high. The 1 1/2" section sits on the rear crossmember and the 3 1/2" section is riveted to the skin.
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