Ah, so your thinking run the bolt through, apply a small o-ring(or rubber seal), a washer then tighten the nut? I would think that should work.
That is a nylock nut with not a lot of threads showing. I'm afraid the added thickness of the o-ring and a washer would shorten that thread contact even more. I think most body guys would put a dab of silicone over the hole and assemble the parts then wipe away excess sealer. It would seal the hole, remain pliable and not add thickness to the parts.
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1979 CJ7, FI 5.0L Ford, NP435, D300, Full floated D44 Detroit, D30 Detroit EZ Locker.