Ok, here is the results of my belly pan fastener situation.
Instead of telling you what didn't work I'll just tell you what did.
I ended up keeping the original "U" clips in the rear and I took a 10-32 tap through them.
I found a couple of s/s 10-32 pan head screws and #10 washers in my screw box and yesssss, the
back of that pan is secure now!
I used one of the original screws with combined plastic washer from the rear along with a "U" clip up at the front center and, yessss tight and secure.
I feel confident now that I can drop that pan as many times as I want and be able to put it back up securely, without stripping anything.