As for L-shape you mounted it upside down. IMHO. Short piece should go flush underneath frame rail and welded on the other side kinda like those cross-member/body mount/trailing arm attachments. This way any lifting loads on rails would try to twist entire cross section of frame instead of trying to deform (expand) long face. Those holes are for plug welds as well.
Here is what I mean:
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or like this depending on size
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