I did these drawings as part of the design process for the pickup.
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The center step section is made from 1/4" steel plate, welded together. The factory bumper has a section cut out of it that's the same length as the step, and the bumper sections attach to the step section with bolts. I didn't weld the bumper sections to the center section because doing so would ruin the powder coat on the bumper sections. All I did was touch up the cut ends of the bumper sections with black paint - I did this over 10 years ago and the bumper still looks great so for my use painting the cut ends was the right way to go. If you wanted more strength you could weld the bumper sections to the center section and refinish everything.
The center section bolts to the rear frame crossmember using flathead screws countersunk into the back steel plate of the center section.
A view of the unfinished center section:
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I did the bumper mod before I cut the LJ in half to turn it into a pickup, so here are some photos of the bumper on the stock LJ. The step is centered and the factory spare mount isn't, so the spare and the step don't line up exactly but that's ok because the bumper on the stock LJ was temporary. If you were doing this bumper to allow a larger spare to sit lower on the tailgate you might want to a) position the step section so it's centered under the spare and b) make the step section wider so the spare could fit lower into it.
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A closer view. You can see where the license plate lights are, but the plate isn't installed yet; that was done when I did the rest of the pickup conversion.
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If there's anything that isn't clear from the above, I'll be happy to provide more information.