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Thanks for posting the photo Super E----helps a lot I think.
I had an '88 E150 that had this same issue and IIRC my mechanic of the time removed the lower bracket that the tray is welded onto. You might want to look under the tray and if possible remove whatever holds the assembly to the body or frame. I'm sure you've already thought about this though!
Once that's out of the van it might be easier to see how it was originally attached. Once its reassembled I'd paint or coat the new tray on all surfaces with a rust inhibiting material so this doesn't happen again too soon.
Sorry I can't be more help---really thought someone else would have contributed by now.
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